Millions of people were taken from their homes and killed. This would leave them deserted with nobody to claim them. What happened to them after the war.
Can only give an explanation about Belgium, some information will be similair with other west-european countries but i'm a bit ashamed since, although I've been a guide to concentration/extermination camps and exhibitions about the subject for the last 9 years, I haven't done any extensive research on the post-war situation in Eastern Europe for jewish people.
So do keep in mind that the following explanation is about Belgium, and even in such a small country the situation is diffrent from place to place.
Although there were around 70.000 Jewish people in Belgium in 1939 only a little more then 5% had the Belgian nationality. The majority were people who had fled oppression in Eastern Europe and, after 1933, Germany. Since they were new and impoverished arrivals most of them didn't own their house but rented a house in one the major cities (Antwerp 35.000 , Brussels 25.000, Liege and Charleroi).
Some 25.000 Jews were deported from Belgium, another 5.000 from Northern France, which was part of the same occupation zone as Belgium for the German administration. Your chances of deporation changed drasticly depending on the city and wether the local goverment aided the German effort. In Antwerp 2/3 were deported, in Brussels 1/3, Liege and Charleroi 2/5.The majority of these deportees were recent arrivals, so less likely to own a house, who didn't have a grounded social network in Belgium. (therefor having less chance of going into hidding) Of those 25.000 jews only a 1000 survived.
After the war refugees (jewish and non-jewish offcourse) could reclaim their property but the 2 deported, Belgian, holocaust survivors I know didn't do it because they didn't want to return to the house that reminded them of the family they lost. Ironicly the 2 non-belgian deported holocaust survivors I know didn't even return to their own country after the war. One was from Kos and lived in Italy for a few decades before coming to Belgium, the other was of german/polish decent and came to Belgium after the war. The other jewish holocaust survivors (who weren't deported) I know their parents either didn't own a house or sold it during the war before going into hiding.