Conservatives weren’t enthusiastic about McCain. And they despise Romney. The people who matter, the party has a difficult time attracting them to a candidate they can’t see themselves voting for. The comfort level isn’t there.
Conservatives despised McCain and even turned to Romney in the end in 2008 to try and stop McCain (at least on FR). There's never a "comfort level" with anybody. Face it, we're all a bunch of stray cats and no attempt to "herd us" is going to bring about any "comfort level" with anyone. ;-)