And because he speaks off the cuff with sincerity (whether you like what he says or not), people trust him.
And because he is an outsider who can't be bought, he is doubly vetted.
And because most of Americans are desperate, I mean DESPERATE, for real leadership (and whatever else he is, he is a strong leader), he is triply vetted.
Hence the wild enthusiasm.
People are permitting themselves to hope that perhaps the country just might not be lost after all.
I think that's it, in a nutshell.