Because the calculus is to run the most ELECTABLE conservative, one that can win. The issue then becomes the interpretation of “electable”. Rick Santorum is not electable IMO. Plenty on FR will disagree with me but I don’t consider Palin electable either. She would be the unappeasables candidate, however. Which candidate would you like to see the GOPe compromise on? Ben Carson would not be approved by the unappeasables.
History disputes that theory though, because whenever we have run the most conservative candidate, they have won. It’s when we go with mushy moderates that we lose.