Because he is right wing and the current system is working in his favor.

The worst thing that could happen to the democrats is a competent opponent. They need trump in order to win so post election you’re so burned up from worrying about hard fascism you become more docile towards the shit were left with.

This is, i believe, why some rich fuckers donate to both sides. They cant purchase a president but they can purchase a party nominee and by making one of two choices completely unacceptable we are forced to take the alternative.