Oftentimes people respond to a crisis by claiming that it could not have been foreseen. (Government officials said this in the wake of 9/11, as one example.) With regard to the housing crisis, I have an answer: Henry Hazlitt. From his 1946 book Economics In One Lesson:
The case against government-guaranteed loans and mortgages to private businesses and persons is almost as strong as, though less obvious than, the case against direct government loans and mortgages [for homes]. … Government-guaranteed home mortgages, especially when a negligible down payment or no down payment whatever is required, inevitably mean more bad loans than otherwise. They force the general taxpayer to subsidize the bad risks and to defray the losses. They encourage people to ‘buy’ houses that they cannot really afford. They tend to eventually to bring about an oversupply of houses as compared with other things. They temporarily overstimulate building, raise the cost of building for everybody (including the buyers of the homes with the guaranteed mortgages), and may mislead the building industry into an eventually costly overexpansion. In brief, in the long run they do not increase overall national production but encourage malinvestment.
Over sixty years ago, and he nailed it. What a shame nobody was listening.