history

People leave their country for a million reasons, but every person who left the Soviet Union got lit up by the propaganda machine so we tend to think that way more people did it than actually did. The Soviets themselves put rules in place to try and prevent certain high-value people from leaving, like nuclear engineers, and this created a lot of friction between a certain class of professional and the state - but taken as a whole Soviet emigration wasn't much different than, say, American emigration today.

edit: East Germany is a different story, along with German-speaking minorities being expelled from Eastern Europe after World War 2. With the power of hindsight I think it's safe to say that the East German authorities mishandled the situation and made things worse for a long time before it stabilized, which is why 1/5 of East Germans left the country - but it did stabilize, with emigration dropping precipitously after 1953 with Stalin's death.