Simple answer: (1) they are cowards; and (2) it would confuse the narrative. The narrative is that conservative Christians are bigots and haters, and putting Muslims into the equation would put a supposedly threatened minority on the side of the bigots and haters. It might provoke a more nuanced discussion, it might mute the hysteria, and that is not tolerable, so it won’t be allowed to happen.
Because they are hedging their bets if Cruz wins the nomination. They are going to try to desensitize the 18-34 vote to a point where hearing the word God will be like pushing grandma off the cliff back in 2012.
Bet on this. Until Cruz announced, not much.