The theory that Romney wasn't conservative enough to win:

[TJ in Oregon]

If Romney had a problem with the conservative side (not necessarily saying he did) it wasn't that he was too moderate, it was that he transparently flip-flopped and therefore forfeited the trust of many of his supporters. If he had consistent moderate positions the whole time he may have fared better in this department.

I'm not really convinced he had a problem with conservatives and I for one still did vote for him. I didn't bother trying to convince anyone else to though like I did for McCain. At least for me it was a rather indifferent Mittens vote. It didn't seem to matter much who won.