Just because they are stupid AND desperate...
They're so desperate for something to change and push the world in the right direction and so tired of the establishment that they would take anything that seems a bit different.
That's a very valid sentiment and one that Donny is riding in an expert manner.
That's why it's hard to have much respect for the democrats for propping up a tired zombie as his rival. Anyone else with a little spark and some originality would've creamed him.