Newt Gingrich has too many skeletons in his closet. His extra-marital affair and divorce of his sick wife was a low. Republicans tend to think that higher morals and ethics are a requirement. Newt is also guilty of a "less than perfect" record on raising funds, and would be a continuous target for Democrats who would like to get even with him.
Don't get me wrong -- I think he is the most brilliant conservative in the history of the party, but he would never be able to win enough votes to become President.