David Brooks correctly diagnoses it: it's his rationality. As Brooks writes:
The most impressive thing about Mitt Romney is his clarity of mind. When he set out to pursue his party’s nomination, he studied the contours of the Republican coalition and molded himself to its forms.
[snip]
And yet as any true conservative can tell you, the sort of rational planning Mitt Romney embodies never works. The world is too complicated and human reason too limited. The PowerPoint mentality always fails to anticipate something. It always yields unintended consequences.
And what Romney failed to anticipate is this: In turning himself into an old-fashioned, orthodox Republican, he has made himself unelectable in the fall.
I think Brooks is right about this. Romney, in his heart of hearts, is probably a fairly unorthodox Republican. And an unorthodox Republican would actually have a decent--albeit not great--shot at beating the Democratic nominee in 2008. But one look at any of the numerous polls showing a generic Democratic presidential candidate trouncing a generic Republican one reveals just how unelectable an old-fashioned, orthodox Republican will be in '08. Hence Romney's problem.
In fact, the GOP's best shot in the fall is probably McCain--specifically a McCain who bears an increasing resemblance to the McCain of 2000 rather than the McCain of 2006. I think this pro-McCain point was basically the sub-text of Brooks's anti-Romney op-ed (which is why, I'd imagine, the McCain campaign is emailing Brooks's piece around this morning).
But it's the point Brooks makes in a brief aside that I think is most interesting--and telling. He writes:
The leaders of the Republican coalition know Romney will lose. But some would rather remain in control of a party that loses than lose control of a party that wins. Others haven’t yet suffered the agony of defeat, and so are not yet emotionally ready for the trauma of transformation. Others still simply don’t know which way to turn. [Emphasis added.]
Which is why my money's still on Romney to win the GOP nomination. It's going to be pretty fascinating to watch Republicans--rather than Democrats, for once--snatch defeat from the jaws of victory.
--Jason Zengerle