So why is he doing it?
In the short term he looks like he is afraid of girl reporter.
Behold the face of Trump-Evil
His opponents get to take free shots at him all night during the debate.
Ted Cruz will have a chance to shine, unmolested by Trump. Possibly giving him a desperately needed point or two in Iowa.
Trump really doesn't do all that well at debates. It's not his medium. It hasn't cost him anything but it hasn't gained him anything in any serious way.
He CAN in fact respond to the shots that are taken at him live on Twitter. This is the first election that Twitter has made any kind of serious impact on. And Trump does do well on that medium. He can assess how well that works for him.
He also gets to assess how big his impact on Fox's ratings is. So does every other network come to that. IF the dip is big enough he has made it clear to the news bots that they need him more than he needs them. And they will need to be more accommodating to his wishes.
Summary: Trump has nothing to lose by not showing up for debate. He is the front runner at the moment. If he stumbled at the debate, he would not have time to recover, so he probably welcomed an excuse to not show up without looking like he was afraid of Ted Cruz. He gets to gather some valuable intel and not endanger his lead in Iowa. He might get to establish a more dominate relationship with the newsbots.
Basically skipping the debate is a very low risk maneuver for him with some potential rewards on the other side of it.