People don't vote candidates, in general. They vote if they are happy about how things are, or if they are not. Usually, is the incumbent (I'm happy about how things are in MY life), or the challenger (I'm not happy, let's change something).
In this particular case the incumbent cannot run, so the proxy is the candidate of the same party. Also, people suspect that the usual challengers are not really a change at all. But in this case it is, or at least it appears to be. So the excitement about it.
Voting or defending Trump has nothing to do with Trump, really, and all to do with a desire for profound change. The people express that desire in the only way that the election game allows them, and that's not a good way, that's for sure, but it's the only one.
You are surprised of intelligent people defending Trump, and I am surprised of how this blatant fact, the desire, of so many people, for many current politics to change or reverse course, is completely bypassed by the media, that chooses to center in the, admittedly rather pathetic, personification of that desire. That's an ad-hominem fallacy if I ever saw one, and you fall into that trap and try to keep the discussion there (the person), instead of on the politics.