From USA Today:

“The moment you start changing the nature of what is guaranteed by the government, what is subsidized, you start to change the alignment of winners and losers. … We took for granted that anyone could get a mortgage.”

Using guarantees and tax breaks, the government pushed homeownership past 69% in 2004. Then it all came crashing down.
. . .
The housing giants (Fannie Mae and Freddie Mac) buy mortgages from banks and other lenders. Usually, they package the mortgages into securities and sell them to investors. Sometimes, they keep the mortgages in their own portfolios. The idea: to create a thriving secondary market in mortgages. By selling their mortgages to Fannie and Freddie, banks clear room on their balance sheets to make more loans, ensuring a plentiful supply and making it easier for home buyers to find financing.