First there are plenty of millionaires and billionaires in debt. Why should the population at large subsidise that lot? Same goes for the large number of well off middle class people living in large houses with mortgages to match.
Second, take two people on say average income or just below. One decides to rent their accommodation and the other decides to get a mortgage and buy. Under a jubilee, the latter would effectively be given a house paid for in part by the former.