The difference is that the Germans were relocating to a country that was more prosperous and developed than the one they left
I hate to be a pedant (LIE) but actually they were mostly leaving parts of Germany that had ceased to be parts of any Germany - including many areas that were once very prosperous and very developed indeed (e.g. Breslau, Königsberg...) - and were relocated to the new West Germany. The exception would be the Sudeten Germans, but then the German parts of Bohemia were also very developed and quite prosperous.
Not that this really has any bearing on the subject of this thread.
Yes, this remains a huge problem (and one that would not be solved by the relocation of the refugee slums to the sites of former villages: slums they would remain). Any ideas?