Good question. He is either afraid to debate or believes it is beneath him. Either way is bad.
The major candidates knew that it was safe to duck out of this debate. No major network or news channel is covering it, so I’d bet the vast majority of “values voters” this debate supposedly represents don’t even know that it happened in the first place.
There will be no measurable repercussions for any of the “big four” whatsoever. That is why they didn’t show up.
He's out on the campaign trail, just like he meant to do when he entered the race. He plans to take part in some of the debates, but not all of them.