Why did the Soviet Union close its borders and restrict travel abroad?

It was in a bid to control the Russian population under the excuse that it was a measure that would prevent Russian state secretes from being leaked abroad.

A colonel from the Federal Security Service (FSB) of the Russian Federation by the name of Alexander Poteyev had earlier defected to the U. S. and revealed identities of a network of Russian spies in U. S. and Britain. After this, no FSB employee was allowed out of the country unless they required urgent medical attention that was not available in Russia at the time.

This exit ban was also extended to ordinary Russian citizens with the exception of highly skilled professionals who were allowed to leave the country to work on projects abroad.