http://www.foreignpolicy.com/articles/2011/01/05/10_percent_unemployment_forever
basically, the jobs were marginally productive and the wrecking of the economy forced those jobs to be eliminated. and they're not coming back.
As time passes, it is harder to avoid the notion that a lot of those old jobs simply weren't adding much to the economy. Except for the height of the housing boom -- October 2007 through June 2008 -- real GDP is now higher than it has been in the entirety of U.S. history. The fact that the United States has pre-crisis levels of output with fewer workers raises doubts as to whether those additional workers were producing very much in the first place. If a business owner fires 10 people and a year later output is almost back to normal, it's pretty hard to make the argument that they were doing much in the first place.
basically, the jobs were marginally productive and the wrecking of the economy forced those jobs to be eliminated. and they're not coming back.