A face to face debate is almost archaic in this day age of so much information being made available through so many various means. Who cares which guy can speak the best about the issues while standing behind a podium? Yes that is definitely Cruz strong suit. But Trump certainly gets his messages across PLUS he has many much more useful skills than talking.
How will Kasich get his message out if he is not in a debate? He is the only candidate who refuses to fight anyone. He does not like anger, hostility and hatred, nor will he feed on it.
Actually, seeing both candidates unfiltered head-to-head would be more illuminating than a thousand internet articles. Trump knows that Cruz would easily cut through his propaganda, and expose him as poorly informed. He can’t risk having the blinders ripped from his followers’ eyes.