He is too libertarian for conservatives and too conservative for libertarians. He is too conservative for liberals and does not carry water for the C of C which means the country clubbers have no use for him.
That's it. Also, if he wants to rise in the Senate as an insider, that means he can't play the outsider card that is so popular this year. Like Bush, Rubio, Kasich and others, he can't capitalize on voter discontent and the hunger for outsiders. If Rand and Jeb were more like their more colorful and unrestrained relatives, they'd be doing better in the polls.