Because he won by narrowly carrying a handful of Rust Belt states that republicans never carry. He got working class white voters in those states to go out and vote for him because they thought he was in their corner, unlike the typical republicans. So now we get a standard, consultant-driven reelection plan involving screaming "socialist" and a lot of flag waving.
These people don't give a crap about the stock market, tax cuts for billionaires or moving the embassy. They do care about their jobs, the opioid epidemic, crime and immigration overrunning their local schools, hospitals and jails. Other than force a few companies not to close plants, he's done pretty much fuck-all on those areas. In terms of these people's lives, the difference between him and Jeb or Hillary is minimal.
They aren't going to vote against him. They just aren't going to bother to vote, just like they did with Mitt and McCain. The continuing tsunami of immigration tilts the field even more against republicans in general. Formerly safe states are suddenly very much in play. Ted Cruz barely won reelection running against an idiot. Illegals are openly encouraged to vote. The democrat vote fraud machine will be in overdrive, after suffering a humiliating defeat in 2016. Social media monopolists are not about to let an army of MAGA supporters do what they did in 2016 either. They have the internet locked down and ready to march in step with the rest of the media.
I hope Trump wins, because I see him as the only thing standing between us and the entire country turning into Mexifornia, but at this point I see it as an uphill battle.