...like many foreigner-critical organizations. I wonder why that is so? Because we lost territory in the East after WWII but not in the West?
No, it has nearly nothing to do with the WWII, but much more with the education and the circumstances after 1945.
When the communists came over in 1945 from Moscow (theis East German satellite was the communist party SED, they bullied banished all "capitalists" -owner of factories, but also free farmers and ranchers- but also the majority of the bourgeoisie.
The result was, that the most of the 2.7 million (of 18 million citizens in that area) East German refugees until The Wall came at the 13.8.1961 were well educated middle and upper class people. The most of the proletariat stayed there.
After 1990 this trend continued. Until now 1.7 million East German (of 17 million DDR-citizens in 1989) has left their country. Again the younger, smarter and better educated ones, especially many girls.
So the translation of "DDR" - "der dämliche Rest" (= "the stupid leftovers") from the 50ies is still valid.
After over 40 communism it was no well-educated and wealthy bourgeoisie left over in East Germany. And its grwoing after 1990 is very weak.
So except in big and university towns on the East German countryside the majority of the inhabitants belongs to the underclass.
Nearly all large Germany companies, trusts, banks and insurances still have their headquarters -with the well paid upper class- in the old West.
In the North half of East Germany there's also a problem with much older roots: also before DDR-times there no rich bourgeosie existed there, no industry, no universities, plus the historical fact, that serfage was usual there in an intensive way until the early 19th century. So there were no free farmers and ranchers neither.
Remains of the common Untertanengeist (servile spirit) are still noticeable.
About the lost German countries after WWII: the 14 million refugees and expellees after 1944/1945 tried to escape for the the Soviet Army and the following communist regimes, so the fled mostly to the Western Occupation Zones = West Germany. So not too many stayed in the DDR.
P.S. To come back to topic: the problem with the parallel societies in Germany is caused by all German governments of the last 65 years. Because all failed in an epic way with their immigration politics.
To be precise: because they didn't made anything. No kidding: even today Germany has not a real immigration law at all