Jim Antle proposes an explanation for why Republicans have been reluctant to adopt a foreign policy of restraint:

Arguments for foreign-policy restraint have failed to gain traction in the Republican Party because of three perceptions of the conservatives making them: namely, that they are hostile to Israel, indifferent to American national security, and naïve about brutal foreign regimes. Paul is aiming to correct these perceptions while emphasizing his common ground with the GOP and the broader conservative movement.

It is possible that these false perceptions could make many Republicans less willing to give these arguments a fair hearing, but I find it hard to believe that this is the main reason why support for foreign policy restraint remains limited among elected Republicans and Republican foreign policy professionals. Most of these people seem genuinely convinced that restraint in foreign policy is dangerous, or they have strong disincentives against dissenting from that view. That’s a product of the continued preeminence of Bush-era figures in the party, the squashing of intra-party dissent, and a failure to re-examine broader foreign policy assumptions in the wake of the Iraq war debacle.

Bush-era figures have an incentive to avoid that re-examination, since it would entail acknowledging their support for a disastrous policy, and it is easier to quash dissent than rethink anything. Because foreign policy dissent still remains more of a liability for new Republican politicians than an advantage, it is easier not to challenge the prevailing views, and that limits the number of realist and non-interventionist politicians at the national level. As Antle observes, many more Republicans now prefer that the U.S. to be less entangled in the affairs of other countries than favor the opposite, but this clear preference for less U.S. involvement is not reflected at the national level or in most conservative media outlets. There clearly is a Republican constituency for foreign policy restraint, but there continue to be significant intra-party barriers that keep this constituency from being fully represented in Washington. Correcting false perceptions about antiwar conservatives and libertarians is useful and desirable, but those perceptions won’t matter as long as most Republican elites are afraid of being identified as an advocate of restraint and realism.