For my money, here's the answer in a nutshell (although Wills quickly points out that Dionne draws a different conclusion from the same evidence):
The truth is that conservatives are right to feel that their own moderates are sell-outs. To be (even moderately) a moderate is to leave the Republican Party—to be what Buckley called an immoral Middle-of-the-Roader. To accept Enlightenment values—reason, facts, science, open-mindedness, tolerance, secularity, modernity—is to lower one’s guard against evils like evolution, concern about global warming, human equality across racial and sexual and religious lines—things Republicans have opposed for years and will not let their own members sell out to. They rightly intuit that there is only one Enlightenment party in America, and the Republicans are not it. That is why they have to oppose in every underhanded way they can the influence of younger people who are open to gays, to same-sex marriage, to feminism.Wills' review of Dionne's Why the Right Went Wrong: Conservatism—from Goldwater to the Tea Party and Beyond is going on the Readings list in the sidebar.
This is the conclusion I come to from a reading of Dionne’s account of Republicans across the half-century story he tells.