To be fair at that time a helluva lot of republicans didn't actually want him, they just thought appealing to "mainstream" was the only way to beat Obama.It took ten whole years for Mitt Romney to go from the most mainstream Republican imaginable - enough that they ran him for President! - to way, way out on the fringes of his party. It really did happen that quickly, and even as someone who watched it in real-time, that's hard to believe.
And Romney's failing was part of the catalyst in just why they rocketed so far to the right afterward. They got collectively angry that even after holding their nose they still didn't win, and so they came to viscously mistrust advice to moderate.
Then Trump won and solidified that mindset that they just don't need to care about anything leftward, even in lip service.
Even though the real story is that Romney could have won if he didn't need to bend over backwards for the fucking loonies in the party. Even with that the guy did fairly well considering Obama had incumbent advantage.