One of the main reasons why the Western Roman Empire collapsed was the invasion and the attacks of the "barbarian" tribes. The Roman Empire and the Germanic tribes had clashed multiple times in the past; however, with the start of the fifth century, things took a turn for the worse for the Romans, as they finally fell under the power and influence of the Goths, the Vandals, and several other Germanic tribes. In 467, the Germanic soldier and statesmen Odoacer took the throne from the Roman emperor Romulus Augustulus and became the first king of Italy. The year of 467 is considered to be the year when the Western Roman Empire officially met its demise.
Another reason was the weak and unstable social and political climate. The inflation and the high unemployment rate, which came as consequences of the constant lost wars and pointless investments in the military forces, as well as the incompetent leadership, caused a lot of problems both for the working class and the wealthy class. Soon enough, the Empire lost the ability to control its economy and its government.
As the Western Roman Empire weakened, the Eastern Roman Empire was gradually rising to power. Byzantium or Constantinople was slowly becoming the biggest and wealthiest city in the Empire and it was constantly protected from outside invaders and attackers, while Rome was neglected. Motivated by the weakened state of Italy, the Germanic tribes transferred all of their energy into conquering the Western Roman Empire.
Finally, some historians believe that Christianity might have actually contributed to the collapse of the Empire, as it contradicted the old Roman traditions and the Church started to gain more sociopolitical power.