Perhaps because they are tired of being ambushed, not allowed to answer questions, talked over by the hosts, and otherwise totally disrespected!
Of course, this is usually the case by the media, trying to be the headline instead of reporting it.
I think there is more to this:
An elected republican who goes on TV and agrees or disagrees with Trump, he/she angers (give or take) half the party, no matter which way he turns.
They disagree, the Trump supporters are mad. They agree, the Anti-Trump supporters are mad.
Trump is not really a unifying figure, one can just follow his tweets and figure that out.
More to gain by saying nothing than saying something.
The no-win cuts this direction,also.