the inability to be able to rent it at a profit usually. If they don't think they can get money out of it, they won't want to. If the economy hits a point where the housing market collapses, chances are they aren't going to want to risk the buy in knowing that they likely won't be able to sell for equal amount.
Or the much faster method: the "scary" government regulating it
A lot of times they're correct. Home maintenance is expensive, if it doesn't get bought up and renovated it'll just end up on an endless list of foreclosures.