Because those were states within the USSR not autonomous regions within those states.
And how did they get into the USSR?
Also under the USSR's constitution they had a right to leave.
Riiight!
And how did they get into the USSR?
Some were part of the Russian empire for centuries. and don't riiiiiiiiiiiiiight to me - I wrote that the USSR constitution was there for show purposes only. Yeltsin's trick to break up the USSR was to use the USSR's phoney constitution as if it was real to break it up.
This is basic History channel stuff - you did not know it?