Why did the Soviet Union fall in the nuclear race?

The Soviet Union fell not in the nuclear race, but in the arms race (mainly conventional weapons). The Soviet Union is ahead of the United States 15~25 years in the nuclear race.

There are many reasons for the disintegration of the Soviet Union, including economic reasons and political reasons.

After the Soviet Union invaded Afghanistan, the country fell into economic difficulties.

The mistake of Gorbachev's new thinking,

Yeltsin opposed the revolution,

The peaceful evolution of western capitalist countries, and so on.

There is a lot of gossip downstairs, that's not what I said.