Historians have debated this question for some time. The consensus for many years after the revolution emphasized the tightly-knit organization of the Bolsheviks. According to this interpretation, the Bolsheviks managed to topple the Russian provisional government with relatively little support from the Russian people as a whole. Lenin, in short, was the leader of a cadre of revolutionaries who, unified in purpose, were able to overcome his opponents, a mixture of royalists, moderate socialists, and republicans who lacked the same will and cohesion that characterized the Bolshevik movement.
While not discounting the importance of these factors, modern historians tend to emphasize the degree to which Bolshevik ideology and popular appeals resonated with the Russian people. The Bolsheviks were especially popular in St. Petersburg, where the people gradually became convinced that only they could be trusted to bring the disastrous war to an end. This interpretation also places great importance on the dire conditions that confronted the Russian people in 1917. Bread and fuel shortages, exacerbated by the war itself, made them willing to embrace Bolshevik reforms. In fact, the Bolshevik program of "peace, land, and bread" was in many ways a response to the demands of the Russian people, especially soldiers and sailors and their families in Petrograd.
In short, the success of the Bolshevik Revolution in 1917 can be attributed to, among other factors, their organization, the conditions faced by the Russian people, and the inability of the provisional government to deal with these conditions.
Good question, but unfortunately it is also a great example of trying to explain the Russian Revolution with a bumper sticker answer. The standard answer will be that WWI created the conditions which the Bolsheviks took advantage of to stage the November Revolution. The standard answer will also talk of the effectiveness of Lenin's message of "Land, Peace, and Bread" in winning over the peasants and working class to the Bolshevik side.
Unfortunately, the answer to your question is not that simple. To fully understand why there was a revolution in Russia in 1917, and actually there were two revolutions, you must look at Russian History going back to the time of Ivan the Terrible. You have to look at historically how the Czars ruled and how they treated the majority of the Russian people.
Closer to 1917 you need to look at the effects of the Russo-Japanese War, the failed 1905 revolution, the unpopularity of Peter II's wife (who was German), the effects of rapid industrialization in Russia, the fact that many nobles wanted Peter the II gone, not to end WWI but to actually fight it more effectively.
So when answering this question, you must look both at underlying and long term causes.
The Bolsheviks successfully made a revolution and seized power at the end of October 1917. This revolution was not just chance; it was planned from the beginning, with several factors contributing to the success of the revolution.
It was these contributions made it possible for the Bolsheviks to seize power.
- The Provisional government was the unfortunate authority that was "in control" at the time of the Bolshevik revolution. This aided the Bolsheviks to success, as they were already very unpopular.
When the peasants tried to claim land, the Provisional government, led by Alexander Kerensky, tried to stop them. This decision and many others, made the Provisional government very unpopular, with the support for the Bolsheviks now growing. With the Provisional government not allowing peasants land and the Bolsheviks opposing this, (Peace, Bread and Land!) the popularity of the Bolshevik party could only grow, and the Provisional governments support could only drop, making the Bolsheviks seem even better, as they opposed the unpopular government.
- One of the most important things that happened for the Bolsheviks was when they defended the Provisional government from Kornilov's revolt.
Kornilov had a larger army with more man-power than that of the Red Guard (Bolshevik's forces) and more armoured vehicles and other assets. Although the Red Guard had a much smaller defence force than that of Kornilov, the Bolshevik force was superior in many ways. The Red Guard were very highly disciplined, with excellent training, which made them better-quality soldiers than the forces of Kornilov. Also, the Red Guard were dedicated to the cause of the revolution, with each one determined to fight to the bitter end, as they were fighting for a cause they each firmly believed in. Kornilov's troops however, were just recruited, and weren't as determined for there cause, as many of them wouldn't have had a cause, they were just fighting because they were ordered to. Kornilov's soldiers weren't too keen on the idea of the assault on the Provisional government, and they weren't all that pleased to be fighting, especially against those who were so determined for their cause, which the Bolsheviks were. Kornilov wasn't that much of a political leader either, described as a "man with a lion's heart but the brains of a sheep," which didn't help his soldiers appreciate the orders he gave them.
- When the Kornilov forces tried to attack the Provisional government the Bolsheviks resisted the attack, defending the unpopular Provisional government. Through the whole of the Kornilov attack, they knew that by defending the Provisional government they would make themselves, the Bolsheviks, more popular.
The Provisional government did not realise this, so they were only too pleased to get a decent army (the Red Guard) to defend the government from Kornilov, who was a major threat to government. The Bolsheviks were also a threat to the Provisional government, but they didn't see this, as they were only too keen to get the Bolshevik forces on their side.
- The Bolshevik party also had two other factors which made them so successful. They had support in the right places. Although they didn't have the support of the whole country most of the country's population was illiterate. This made most of the peasants, who were the ones the revolution was partly for, unable to understand what was happening, whether it was good for them or not, as they couldn't understand what was going on.It didn't matter though as the Bolshevik party had their support in the right places, where it was significant. Over half the army supported the Bolshevik cause, and with the army, nothing could stop them.
This was because now that the army was on the Bolshevik side; the authorities couldn't enforce the measures necessary to put down the revolution, as the army was no longer on their side. Also, with the army the Bolshevik could control the communications and travel network throughout the country. The army took control of the railway network, the communication lines and other means of power, making it nearly impossible for anyone to stop the revolution.
- Kronstadt, the main Russian naval base also joined the Bolshevik cause.
This was a huge boost for the Bolsheviks, as now they could control the seas with most the sailors joining the Bolshevik cause they were now also able to control what was coming in and going out of the country. Also the naval cruisers such as the Aurora were now in Bolshevik possession and could supply heavy fire several miles inland if necessary.
- The major industrial centres in Russia also joined the Bolshevik cause.
This meant that the Bolsheviks could now manipulate the Russian industry, they could make ammunition and all the goods could be available for the Bolshevik forces, if they need them. As they could manipulate them, they could produce all the materials they needed to make certain things e.g. buildings, weapons, propaganda. The Bolsheviks lastly had the support of the Petrograd and Moscow Soviets. This was especially important as the Soviets had quite a considerable amount of power, and also were against the current living conditions faced by the peasants, which the Bolsheviks were also against, making a handy devotee for the Bolsheviks, as they were both for the same cause.
- The last major factor which made the revolution possible, were the heart of the Bolsheviks Lenin and Trotsky.
Lenin was responsible for many things, one of which was his propaganda. It was Lenin who produced his April Theses, including his political ideas and beliefs, which when read by others, encouraged them to take action and side with the Bolsheviks. Described by sources, Lenin "was the overall planner of the revolution he provided tight control, and a degree of discipline and unity which the other parties lacked." This source particularly shows how Lenin was seen as an influential leader, making the whole revolution possible, using his great leadership skills. He had faced exile in Siberia, yet he still managed to hide away, and return with influential speeches and propaganda, making him a superb and prominent leader. He was the person who planned the whole revolution, taking action against what he believed strong about, finally leading the Bolsheviks to power.
Trotsky was also another influential leader, who led along side Lenin.
When Lenin and Trotsky were exiled Trotsky escaped in 1907, and tried to bring the Bolsheviks and Mensheviks together, always working with Lenin. When the Bolsheviks took power, Trotsky was put in charge of the post offices and communications, bridges and the State banks, the most vital areas of Russia. Lenin trusted Trotsky and let him take the responsibility for those jobs.