Let me turn this around by saying “Why can’t Trump win by getting a majority?” Means over half of all voters prefer someone else, doesn’t it?
So far, those have been few and far between in the states primaries and caucuses.
Trump does not need someone else to drop to win based on math. If the voters don’t choose him (and apparently they are) then he loses. Cruz needed Rubio to drop, or now it’s Kasich, to “make a path for him”. If Cruz was as brilliant as everyone has said he was (even himself) would be not be in the lead?