Why does Obama let Romney get away with his "doubled the deficit" charge?

Obama did a lot of things right in the Hofstra debate. But one core Romney lie he let go unchallenged repeatedly. I go to my tweet tape:

@xpostfactoid1 

nailed him on everything but his own deficits

6:51 PM - 16 Oct 12 ·  Details

3x "he doubled the deficit" - time to pop that lie
6:56 PM - 16 Oct 12 · Details

he has cut the deficit - time to say so

6:57 PM - 16 Oct 12 · Details

unaswered: you doubled the deficit Lie
6:37 PM - 16 Oct 12 · Details

In fact, the deficit in FY 2009, compiled mainly on Bush's watch, was $1.4 trillion; this year it's just shy of $1.1 trillion. The only way Romney can get to Obama "doubling" it is to hold him responsible for 2009 -- as he's held him responsible for job losses that occurred prior to the stimulus taking effect (while holding himself harmless for job losses in the first year of his Massachusetts tenure -- see Romney Rule #2).