At his advanced age, and in his poor health, he really shouldn't be doing anything strenuous like that, nor anything stressful like campaigning for office. Maybe that's why he skipped the debates; his heart couldn't handle them.
He is leading by a large margin, the debates at best won't do anything at worst would cause him to lose the lead and help one of the worse performing candidates.
A decent candidate would still take part in debates, because it helps Americans to make a more informed decision, but Trump isn't decent.
Actually going to the debates will show to everyone how much his mental acuity has declined, and take some of the wind out of his "Biden is senile" comments.