The perception about Republicans in general and candidates in specific is that they don't really like to talk to--or listen to real people (or press people who might represent them). Might dirty their hands to have anything to do with anybody who can't bundle millions or deliver a megachurch of voters. And it might hurt their campaign to take real questions not scripted in advance (sort of like Bush's idea of congressional testimony--no transcript and questions written by the White House). So I do agree with you that it would be good if candidates would participate and be seen to be engaging with real voters.

That said, as a registered Democrat, I hope they don't :-)