How would Trump ever get above the 25% he got here in a state like Iowa, if Cruz stays in? Which voters of Rubio, Carson, Bush, Christie, etc. would switch to Trump if they dropped out?
The reality of the overall polls is that Trump can never get over 50% even in one-on-one matches. Someone who only appeals to a minority of Republicans is a probably loser in the general election.
He was not expected to win here. Winning here would have been a cherry on top.