In the fall, I had several posts on why conservatism became a "movement." I've been thinking about this issue off and on since then, and was reminded of it by a recent post from Paul Krugman. My explanation is:
1. A political tendency that has the major institutions of society on its side doesn't have to worry about doctrine--it can just appeal to "common sense." A political tendency that is excluded from the major institutions of society has to develop a doctrine and institutions to support an alternative vision. For example, in the late 19th and early 20th centuries, labor and socialist parties in Europe established a network of alternative institutions: newspapers, publishing houses, adult education classes, and even social clubs.
2. In the 1960s and 1970s, liberalism came to dominate education and and what was then called "the media" and what would now be called the "mainstream media." Now that conservatism was in opposition, it became a "movement." Since conservatism continued to be strong in business, it was a well-financed movement.
3. The shift in the political leaning of the media and higher education seems to have occurred in many or most economically developed nations, but the change of conservatism to a movement was unusually strong in the United States. Why? I proposed an explanation based on national differences in values, but it didn't fit the facts very well. Then it occurred to me that the difference might be political institutions: the system in the United States makes relatively easy for outsiders to mount challenges. In most countries, the party selects the candidates for parliament, and the parliamentary party selects its leader. That is, in order to get ahead you have to win the approval of people who are already on the inside. In the United States, you can mount a challenge, win a primary, and push your way in, as Marco Rubio did in 2010, Ted Cruz did in 2012, and Donald Trump is doing now. Successes help to sustain a movement.
4. This explanation raises a question: why have the successful challenges in recent years all come from the right? The things I talked about in points 1 and 2 apply to the minority who are deeply engaged in politics: most ordinary voters are not consistent conservatives or liberals. The image of taking a stand on principle has some popular appeal, but so does the image of not caring about ideology and being willing to compromise to get things done. So why don't Republican primaries see strong challenges from the center?
5. One factor is psychological: it's easier to be passionate about taking a stand on principle than it is to be passionate about considering all points of view and taking things on a case-by-case basis. Another is historical: the major advances of the Republican party since the 1970s have involved shifts to the right. There was Reagan in 1980, Gingrich and the "Contract with America" in 1994, and the Tea Party in 2010-4. So an ambitious newcomer (e. g., Marco Rubio) can reasonably conclude that the way to get ahead is to keep pushing to the right, and an insider who doesn't really care about ideology can conclude that "no enemies to the right" is the best policy.
I don't have any real evidence for it, but I think that the historical factor is more important than the psychological, so that pretty soon (maybe after the 2016 election) the moderates will start successfully pushing back. But I think that the more basic pattern in which conservatism is a movement and liberalism is a loose coalition, will endure for a while.