After the Germans that had been there were kicked out.
Upshot? The Soviets made sure the eastern border was "clean" by claiming the mixed Polish/Relorusian/etc. territory in the eastern 100 miles of the country for Belorus'. Those ragged ethnic borders that population movements and intermarriage/assimilation produce were "rectified." Much like France's were, by the same means.
And the same was done for the western border. Who knows what's up on the southern border, but after WWII a number of E. European countries engaged in serious assimilation. Lots of Slovaks in Hungary were forcibly assimilated (while the more tolerant Czechs/Slovaks didn't require the Magyar in S. Slovakia to do the same, to their chagrin). The Poles also did a bit of assimilating, since the southern border with Slezsko and Cechy, two of the three part of the current Czech Republic, didn't have a clear ethnic boundary. The dialects there are clearly Polish or Czech, but then again the languages aren't *that* different. (To the point that after being around mono-lingual Czechs for a couple of months back in '94 or '95 I was merely annoyed that the Czech movie I thought I was watching on Los Angeles' foreign-language channel at the time wasn't as comprehensible as I'd hoped. I thought it was in Czech, but it turned out to be Polish. Between Russian and Czech ... the war movie wasn't gibberish.)
Nobody wants to immigrate to Poland. Well, I might, but it's not on the top of my list. I've spent a month there, nice enough place, even Lodz.
Otherwise I have no problem with an indigenous population remaining almost entirely indigenous (taking into account all the population shifts, that is). I think that people have a complaint about it because they don't like the domestic politics.