1. has team support. For many republicans, it simply does not matter how unprepared he is - there is an R next to his name, end of story.
2. has "frat" boy support. Certain personalities love alpha male bluster. His tendency to upgrade his trophy wife every decade or so and bling his name everywhere is seen as a sign of strength.
3. has the racist/bigot crowd. From the blatant David Duke to the person who starts every bigot rant with "I'm not a racist, but..." These folks get all warm and fuzzy every time Trump announces, "I am NOT politically correct!"
4. has the xenophobes. America is a country of immigrants where many fear the latest batch of immigrants.
5. has the bumpkin vote. Trump will get the vote of every moron who resents people who have a high vocabulary. The more ignorance he shows, the more they like him - he speaks their language.
Any of these groups, on their own, do not have a lot of voting power. But for the first time, we have a candidate who can appeal to all of those subgroups. Their combined awfulness may just get him in the White House.