Why have lending standards tightened so much?

Supposedly the US credit crunch is over, but ordinary borrowers still have difficulty getting access to credit.  David Wessel has an interesting WSJ column that shows how higher credit scores are now needed, even among those with steady jobs and a good credit history.  This keeps buyers out of the home market and makes it more difficult for those with homes to refinance. 

While no one wants to see a return to the days when a person could get a home loan for zero percent down, one has to wonder why lending standards have swung so far the other way.  Many of the people Wessel talked to pointed their fingers at our old friends Fannie Mae and Freddie Mac.  Burned by so many bad loans, have they now become too risk averse?