The primary reason for this is that there already has been a civil war in the Republican Party; and the far right won. The situation in the Republican Party today is very different than at various points in the 20th century when liberal or moderate Republicans fought with right wing for control of the party. Today, there is no battle between the moderate and far right wing of the Republican Party. That fight ended years ago with the moderates losing. Although there may be a small handful of centrist Republicans remaining in the party, they do not have a lot of power and move quickly to the right as they become national figures. The most recent obvious example of that was Mitt Romney who had governed Massachusetts as a moderate, but ran for president as a unapologetic conservative. Even people like Chris Christie who are said to represent the moderate wing of the party are, in many respects, conservatives who either have had to govern or who come from the northeast.