What made them "swallow" the ones they did swallow though? Were the swallowed ones already more "Russian" at the time? What is the big difference between say, the Ukraine and Poland? Latvia and Romania? Georgia and Hungary?
The answer is: Russian Empire.
Apart from Poland, all the other territories were Russian not long before !
And even in the case of Poland, Stalin pushed it far away towards the West and grasped back a big part of Czarist territories there.
And Xahar is right. Putting inside the USSR East and Central European countries would have resulted in a dangerous rise of nationalism. Even a part of the left (I mean, socialists who let themselves being swallowed by new regimes), even some communists would have opposed it.
Albania would have been too far away.
Yugoslavia would of course have refused this.
And in East Germany, that would have been really too much for the West, and even for the East Germans.
It was far more efficient to keep formal independence, because the point of "patriotism" worked well to appease some tensions and delay some internal oppositions.
Think about Ceausescu and his nationalism in the 1960s-1970s, which wasn't a real problem for the USSR, but was very efficient to keep a straight control inside the country.
Think about the way Gomulka or Kadar managed to reduce opposition with "patriotic" arguments. Even Jaruzelski used it to justify the martial law.
The only countries where it might have worked for a while: Bulgaria and a separated Slovakia (because not being with the Czechs would have been a small compensation for losing independence).