Because Trump doesn’t have to play by the rules. He has already won 6 debates - Fox went over the top with the press release - so why shouldn’t he stick it to them. Trump is living in their heads and will stay there all through the debate. Let Fox bring out the internet Muslim babe to question the remaining candidates.
Trump "won" the debates the same way Ron Paul "won" the straw polls and debates he was in. I give those online snap polls the same level of credibility.