During the Crash the Fed should have let things fall. It would have been painful. Very painful for some. But the market would have cleared. Some people would have picked up deals and many people would be in houses now, even many who would have hurt in 2008/2009, with positive equity.
Instead the Fed intervened and tried to put a bottom under house prices. As such we have the situation we have now. Millions of homeowners still underwater and a much less mobile workforce.