I wrote over at Greg's place yesterday that one reason social conservatives haven't opposed Mitt Romney effectively might be that they don't really mind if he wins; after all, there's no actual separation on social conservative issues between where Romney says he is and where the other candidates are. Jonathan Cohn made the case yesterday that on fiscal issues Romney is actually quite radical, and Ezra Klein notes that Romney is far to the right of George W. Bush's 2000 campaign.
Indeed, as far as I can tell there's virtually no separation between Romney and the other candidates on core conservative issues. The main "problem" with Romney from their point of view is not his current professed positions; it's whether he can be trusted, given his record in Massachusetts (including both his time as governor and his two campaigns). Note that every attack against him (health care, abortion, his business record) from primary opponents has been about his past, not his current positions. And it's probably a plus for him that whatever his past, he's stuck with his current issue positions (except, perhaps, where the party as a whole has shifted) through two presidential cycles. It doesn't guarantee he'll be loyal in office, but it presumably can't hurt.
This helps explain why the supposed "anti-Romney" vote never really existed, at least to any large extent; it was always a "not enthusiastic about Romney" group (and see John Sides and Lynn Vavreck for confirming data).
Which doesn't mean that Romney was never vulnerable. I think he was. But it does explain why he wasn't immediately toppled by a Tea Party candidate. Team Romney made sure from the start that there just wouldn't be any issue on which a Tea Party candidate could find any separation in their current positions. It appears that it worked at the group level: no key Republican groups really tried hard to veto Romney. And that probably was enough for him to win the nomination fairly easily.