[thriftwy]

Why it didn't happen in 1949 then, or in 1959, or in 1969, or in 1979? What have changed?

I can see a clear link between perestroikas and ethnic cleansings, but perhaps you can reason that away.

[pvg]

It started in the 70s, which is in the Wikipedia page. Bulgaria did not meaningfully have 'perestroika' which you can google your way to. I don't think I have to reason anything away if you're simply unfamiliar with the history - you can just look it up or at a minimum, read the links we're talking about.