One problem has been the fact that some regions of the nation are free of the ravages of the Great Recession, and have healthy employment numbers with sizable job markets... but lack the numbers of people willing or able to live in those markets and fill those jobs.
Yglesias' article on the matter jumps straight into the numbers:
In the metropolitan area centered around Yuma, Ariz., the Bureau of Labor Statistics believes that the unemployment rate is a terrifying 31.8 percent. Just a bit west is El Centro, Calif., with America’s second-highest jobless rate—23.6 percent. Yuba City and Merced, both also in inland California, come next, with unemployment rates of over 14 percent.