Because he faces a field that (other than Cruz, Santorum, and on some days Walker) wants to pretty much dissolve the borders and who won't oppose Islam.
I would like Cruz.
Walker was my candidate before he started stumbling leftward on immigration. Santorum is way past his use by date.
I can hold my nose and vote for Trump.
If Jeb *Jebito* drops out where do you think his supporters will go?
What is in the mind of a Jeb supporter anyway?