Dive summary:

• Irksome as it may be to builders and buyers, banks have a logical reason to be less than liberal in making new mortgages: The banks fear loans they sold will come back to hurt them if the loans go bad.
• Freddie Mac and Fannie Mae have been rigid about making banks take back loans that they made during the housing boom and that later failed, sometimes years later.
• The reluctance of lenders to trust the government entities to keep loans they buy from banks may be a drag on the Federal Reserve's efforts to spur the economy.

From the article:

A battle over who gets stuck with tens of billions worth of bad housing loans made during the boom years explains why many Americans still can't get a mortgage as interest rates hit a new low. ...