Looks like the libertarian wing of the party is becoming more powerful than the conservative evangelical wing, or at least the candidates perceive the libertarian wing to be more powerful. That's probably why they didn't talk about SSM.
They didn't talk about marriage equality because the one thing they can all be counted on to do is WATCH THE POLLS.
And the polls say that a majority of Americans want to join the rest of the civilized world in making marriage legally available to consenting adults, regardless of sex or gender.
Its opponents are just now starting to finally realize that they are on the losing side of this, with 37 out of 50 states plus Washington D.C. recognizing a couple's legal right to marry, and Missouri respecting marriages licensed in other states.
They have been digging a hole for themselves for generations on this issue, and they know that any attempt to backpedal will just dig them deeper in the public consciousness.
The nation is finally breaking away from its bigoted past on this point, and those who have made a career of fighting for that bigoted past now have nowhere to go and nothing to do but wait to be ground under the wheels of history.