Because he took the foot off the gas and should have spent his time in MS instead of AL. Keep it to the floor until 1144 is reached- period. Plus turnout was far too low where he needed it, even if the county targets themselves were reached.
Romney has to take into account that his destructive tactics of negative advertising lowers turnout.
This is a big factor. I've mentioned it elsewhere, but Santorum has a "call from home" program that allows you to call selected voters in battleground states, and I've done it a few times. Whenever I called Alabama the people I talked to said they were getting 10 to 20 calls a day from the Romney campaign trashing Santorum for being pro-choice and pro-gun control. Most of them that I talked to were going to vote for another candidate (Santorum, occasionally Gingrich) or stay home because they were sick of it. Some of them were even Romney supporters to begin with, but were turned off by his attack ads and robocalls.
I know it's anecdotal, but it's something the Romney clowns should take note of. They think they can get away with launching nasty and bogus attacks on their opponents while Mittens goes on TV and talks about how he's the one being attacked. I get the feeling it backfired big time in MS and AL...