Why the soviet union collapsed?

It's believed that the oil crisis of 1979-1982 has been one of the reasons responsible for the collapse of the soviet union as it led to a severe financial crisis that impacted the union. The soviet union depended on selling oil for income. 

The soviet union promised its citizens a great life but the failure to deliver those promises and to out perform the west made many people unhappy.

Many of the non-Russian ethnic groups, which compromised more than 50% of the population,  failed to blend in.

As communism failed to deliver its promises it lost its influence upon people. This created the desire for many people to find a different system.

By the time Mikhail Gorbachev rose to power the union was already suffering of severe economic problems. Those problems accelerated the collapse of the union. (See How War Affects Economy?)

Mikhail Gorbachev implemented some reforms but still failed to improve the deteriorating political and economical conditions of the union.

The Glasnost policy was a policy implemented by Mikhail Gorbachev that encouraged freedom of speech and transparency but it backfired as people started expressing their anger towards the government.

Many states believed that they can do better on their own and as a result they followed the steps of Estonia.

As states became more and more angry at the overall situation a nationalism movement rose that gained momentum and that demanded Independence for many states.

When Estonia demanded autonomy it was shortly followed by other states who believed that they can fare better on their own.

In a last attempt to save the union the communist made a coup and captured Gorbachev but shortly the coup failed as the military refused to shoot the demonstrators on the streets.

As the soviet union went into an arms race with the rest of the world , and specifically the united states, many resources ,including great scientists, were put into the army thus affecting the technological advancement of the union.

The soviet union was large that it was hard for a centralized government to manage it it efficiently. 

As the democratic movement became stronger ,after the USSR decided not to intervene in eastern Europe, Poland announced it's first non-communist governor.  Shortly many European countries followed the footsteps of Poland.

As the soviet union got involved in the war in Afghanistan more pressure was put on the already stagnating economy. (See Why Did the Patriots Win the Revolutionary War?)

Some people believe that the Cherynobyl Disaster was among the reasons that led to losing trust and faith in the soviet union by the different states. 

The soviet Constitution provided legal ground for states to freely secede from the federation. The states used that right and demanded independence.