I think it is simple. Trump has reached a point of popularity that he does not need the debates. There is no value added effect for him by attending. All he was doing was setting himself up for the others to increase their popularity by taking pot-shot at him.
It was a simple decision. There is more risk of a negative consequence than a positive outcome by him subjecting himself, in this case in particular, to a possible setup. Ergo, I'm not going. I see the logic in what he is doing and it makes sense.
He has now become a force unlike any of the rest of the contenders. The event tonight will seal the deal on Trump being a movement unto himself more than he is a political candidate.