Ezra Klein thinks Romney's ideological reincarnation is the main reason GOP voters aren't supporting him:
Whatever Gingrich’s heterodoxies, conservatives never worry that he’s not, on some fundamental level, a committed member of their tribe. He’s an odd member, maybe. A member who has unexpected ideas about the moon, perhaps. But a member. With Romney, they worry about it constantly.
Chait considers the Democratic party's perspective:
Democrats fear Romney precisely because his history suggests he will try to escape the GOP primary with minimal ideological commitment and then run to the center, and that his progressive record in Massachusetts will help him appeal to moderates. Obviously they would rather run against a Republican who’s more heavily encumbered by actual beliefs. For the same reason, Romney is the one Republican (also save Huntsman) they'd most want to be president.