It comes down to two things: consistency of brand, and constantly creating content. Media does the rest. If you think about 45, he almost always wore the same thing, put his name on everything, applied the same style of opinions to everything (either loved or hated something, with limited nuance). Offered a simple opinion on everything.
He was always slightly outrageous which the left wing media had a job dealing with in a way that did not benefit him. He'd say something a bit bad which was then on the cover of all the newspapers when it would have been politically better to ignore him.
This is a solid point about Donald Trump. I owned the Trump board game as a kid. (Released in 1989! https://en.wikipedia.org/wiki/Trump:_The_Game) We thought he was such a goof, but he was definitely a social media star before social media networks.