In short, Romney lacked roots in both Washington and at the grassroots level; he essentially foisted himself onto the party.
As much as I would like to characterize Romney as an unwelcome interloper who forced himself on his supporters, this lets the latter off a little too easily. If there was a time when Romney foisted himself on Republicans, it was in 2006 and 2007 when he was organizing his first campaign. Back then it would have been fair to say that he was trying to impose himself on a majority of the party with which he had almost nothing in common until he started running for the nomination. However, by the end of his first campaign, Romney had been embraced as the candidate of large parts of the conservative movement as their preferred alternative to McCain. Most Republican primary voters weren’t buying his act at the time, but many activists certainly did, and they propelled him into the position of being the effective runner-up and likely favorite for the nomination in the next election. Romney probably could not have become the nominee this year without the support he received from movement conservatives in 2007-08.
During his second run, Romney seemed to realize the limits of being the movement candidate. Romney had the advantages of being the front-runner and having run once before. It’s true that there was no great enthusiasm for him from party leaders or activists in the 2012 campaign, but in the end he was viewed favorably by an overwhelming majority of Republicans and strong resistance to him was limited and divided against itself. One of the weaknesses of anti-Romney forces in the primaries was that more than a few of his critics, including some of his fellow candidates, had been his supporters in 2008. Another was that the opposition to Romney never settled on any one alternative and kept jumping from one candidate to the next. Even when they were openly against Romney, movement conservatives’ inconsistency and indecision worked in his favor. Movement conservatives made Romney a viable candidate inside the GOP last time, and then completely failed in their half-hearted efforts to block him this time.