The USSR was the de-jure federation, de-facto - unitary state. There were no restrictions, people lived there, where they wanted to, because the standard of living differed slightly, salaries differed only by climatic zones (in south paid less in the Arctic - more, but prices were different too), rather than administrative boundaries.
If in 1922 won the unitarian project of Stalin, all Soviet republics would be autonomose territories within the RSFSR. But the Lenin's federation project won - that planted a bomb under the base of the state.
History has yet to assess the traitors - Gorbachev, Yakovlev, Yeltsin, Kravchuk and others.
In fact, Russia wanted to keep nuclear weapons in the former republics. This would exceed the limits of treaties and have additional arsenals, as the US has in France and Britain.