There is a simple reason that Ronald Reagan could explain Conservative philosophy so clearly - he believed it. The same is true today of real Conservatives like Sarah Palin, Marco Rubio and Daniel Hannan. It's not necessarily that they are gifted orators, but that Conservatism is "in their gut" and therefore not difficult to explain.
The opposite is true of fake Conservatives like Dole, Bush, McCain and Romney. Their tortured words belie the simple fact that true Conservatism just isn't in them. As Palin said, "It's either there or it isn't."
Something else: Palin is the best stump speaker among national political figures. Rubio is a better speaker, over all, as he can do both formal and informal.