Question

why was the soviet union dissolved

Answer 1

To tell it in an extremely simplified way, the various territories of the Soviet Union broke away from the Union due to economic and political pressures.
There is a lot of history and background spanning the majority of the 1900s, but I think most of the information about the collapse of the USSR took place from 1950s onward, which you can read about here if you'd like: https://www.history.com/topics/cold-war/fall-of-soviet-union (specifically towards the end of the article)