Loads of reasons but ill give you a good one, hes a complete misogynist, everyone knows it, and there are more women in the us than there are men, more women actually go vote than men do, and then even men with brains will understand that his going off at the mouth is a security risk.
He was asked about it by megyn kelly and instead of a decent answer, she was accused of ragging. Then he still couldnt get over feeling like she made him look bad and kept after her. What a crybaby loser.
In the real debates when it gets more intense, im sure his thin skin will come out even more.