The main reason, I suspect, is that the Republican Party is extremely unpopular. The Bush years deeply discredited the GOP, and while Republicans were able to make gains in 2010 by default, as the out party during an economic crisis, they did nothing to rehabilitate their image. Indeed, they have embraced even more unpopular positions than the ones that George W. Bush advocated. Romney has taken up the banner of cutting Medicare in order to make room for lower taxes for the rich, and that’s an incredibly unpopular trade-off.
Douthat admits that Romney has made mistakes:
The problem wasn’t just that Romney kept putting his foot in his mouth with comments in the "I like to fire people" vein. It was that even when he wasn’t blundering, he didn’t have clear and fluent answers to entirely predictable questions about Bain Capital’s business practices, his own taxes and investments and other wealth-related subjects. Presumably the Romney camp didn’t expect these kind of questions until the fall campaign, but that’s a poor excuse for a candidate who’s been effectively running for president since 2007.
More on Mitt's weakness here.