Even if you ignored him routinely acting like a giant twat, it's still hard to see how anyone can still support him.
If you're talking about voters, there's a conversation to be had here. If you're talking about people like me, ie foreign observers, well then the answer is democracy. Had Clinton won, I would've feared for the future, but if we were here today, and she still had the support of the people who voted for her, which represents a democratic majority, then I would "support" her in the sense I would recognise she is fit to be President and generally avoid insulting people simply for actively supporting her.

Why do voters support him? idk, would I if I could vote for him? I certainly would have done at the last election, because of who he was up against. I imagine he still has core support because the alternative is perceived as worse. Trump supporters would prefer to see an idiot in charge than a liberal. That's a political choice that should be respected, but we no longer respect each others' right to have different political opinions. This is the new millenium, the age of "I'm right, you're wrong, I'm a good person, you're a disgusting person".