very simple. they lent good money to lowlife, who overpaid for houses they could not afford even at a FAIR price.
these lowlife are now defaulting on their debts, thats what lowlife do, thats why they are called lowlife, and you dont lend to them.
lowlife needs to rent, and be evicted when they default on that. now that they have stolen your money, you are up shits creek.
other reasons include fraud, Mozillo, fraud, greed, fraud, and other theivery too heinous to mention.
this is the best explanation you will get.
Note that few if any will blame the defaulters, preferring to blame ONLY Wall St. This is proof that the average American is a nitwit.