The collapse of the Soviet Union was caused by a number of factors. The general idea, though, is that the Soviet Union collapsed because its communist system was not able to keep its people prosperous or happy.
The Soviet system gave its people very few rights and freedoms. It was a totalitarian society in which obedience to the state was expected at all times. This might have been sustainable if the people enjoyed material wealth and were, therefore, relatively happy. (We can see that China has, so far, been able to remain viable with an authoritarian state because it is experiencing economic growth.) However, the Soviets were not able to provide material wealth. Their inefficient economic system was able to keep up with the US in military terms, but was not able to provide consumer goods while it did so.
Because the Soviet economy was stagnating and people were unhappy, it was clear that reform was needed. Mikhail Gorbachev tried to provide such reform, but people were, by that time, fed up with the system and the whole edifice collapsed when he stopped using the power of the state to suppress dissent.