Where are the half-million units?

One of the many useful things the people at the St. Louis Fed do is maintain the FRED page, which has a cornucopia of economic data that is easy to download and graph.   Tonight I just happened to be curious about this picture:

The red line is permitted housing units; the blue line is completed unit.  Let's assume that there is a lag of one year between permitting and completion (the length of the time series means that any reasonable lag assumption should be innocuous.  If we look at permits going back to 1967 and ending one year ago, we find that slightly more units were completed than permitted (this is OK, because small places do not necessarily report building permit data to the Commerce Department).  But since 2008, about 500,000 fewer units have been completed than permitted.  This is about a six percent melt from permitting to completion.

So the question is: why?  Very curious about this.