Peter Wallison has the answer:
Beginning in 1992, the government required Fannie Mae and Freddie Mac to direct a substantial portion of their mortgage financing to borrowers who were at or below the median income in their communities. The original legislative quota was 30%. But the Department of Housing and Urban Development was given authority to adjust it, and through the Bill Clinton and George W. Bush administrations HUD raised the quota to 50% by 2000 and 55% by 2007.
It is certainly possible to find prime borrowers among people with incomes below the median. But when more than half of the mortgages Fannie and Freddie were required to buy were required to have that characteristic, these two government-sponsored enterprises had to significantly reduce their underwriting standards.
Fannie and Freddie were not the only government-backed or government-controlled organizations that were enlisted in this process. The Federal Housing Administration was competing with Fannie and Freddie for the same mortgages. And thanks to rules adopted in 1995 under the Community Reinvestment Act, regulated banks as well as savings and loan associations had to make a certain number of loans to borrowers who were at or below 80% of the median income in the areas they served.
