There are three points I would make to answer this question.
First, we should not be surprised that Stalin started as “an ordinary revolutionary” and then rose to power. Hardly anyone starts as a leader in any situation, particularly not in a revolutionary situation. When you have a complete overthrow of the way a government and society works, you have to expect that people who were “nobodies” would be able to rise to positions of power rather quickly. It would be very strange if people who were prominent under the Tsar became important leaders after the rise of communism.
Second, Stalin was clearly a very brutal man. He was willing to do whatever it took to get power and to keep power. We can see, for example, that Lenin had started to rethink having Stalin in a position of power by the early 1920s. However, Lenin’s poor health and Stalin’s willingness to use underhanded methods kept Stalin in power. For example, Stalin formed an alliance with Grigory Zinoviev and Lev Kaminev to oppose Leon Trotsky, who was his main rival for power. Stalin would later dispose of Zinoviev and Kaminev when he no longer needed them. Stalin was able to rise in part because he wanted power and was willing to do just about anything to get and keep it.
Finally, Stalin was really good at working behind the scenes. He used the powers of the positions that he held to gain allies for himself. He excelled at getting people to feel that they were in his debt and that they depended on him for their own positions. This meant that Stalin was able to build up networks of people who would support him in his bid for power.
All of these factors help to explain why Stalin was able to come to power in the Soviet Union.