Trump didn’t participant in the debate because of Roger Ailes childish press release mocking Trump. He was about to do the debate before the release
Trump was being a crybaby about Megyn Kelly because she was mean to him long before the rather humorous email from Fox. Look at the million threads devoted to it long before the Fox taunt at Trump here on FR. Trump was putting up polls on whether he should go or not, demanding Kelly be replaced, saying he probably wasn’t going, etc. THEN Fox called him out and Trump decided to hold his breath and stomp his feet.
The timeline is quite clear.