Because even conservatives are more inclined to vote for the devil they know, rather than the devil they don't know.
Also, the same party machinations that propelled liberal Mitt Romney to the nomination, are the same as what maneuvered Perry to the front of the Republican pack, time and time again.
Frankly, we were stuck with the guy, and had nowhere else to go. It was better than staying home or turning the state over to a Democrat.