Because he undermined the election process in a serious way. If he loses, it sends a signal that siding with someone who does that is risky. If he wins, it opens the door to someone much more cunning than him, to do the same thing, but better. Its a brilliant way to short-circuit the entire political system which has let the US enjoy such a brilliant stretch of stability.
Let's put Trump aside for a moment. What do you see a Harris presidency looking like? Do you think she is our country's best option to solve the problems we face today, or is her role to fill the chair so Trump can't sit in it?
Harris will be fine but unremarkable. Middle of the road economic policy, with slightly more protectionism for high tech. Immigration policy continues as-is, with a slight increase in preventing illegal immigration. An Obama-like foreign policy, with good intentions, but bumbling in execution.
Compare that to Trump's recorded: Exploded deficits due to tax cuts. Banning abortion. More power given to corporations over people. Harming our alliances. Maybe you disagree with this characterization, but so many people that he chose for his administration came out and said how terrible of a President he was.
Sounds like we just have a different expectation a Harris presidency, nothing wrong with that.
I don't actually know what her economic policy would be, a primary sure would have been helpful there. If it's a continuation of the current policy, I'm not sure how long they can kick a can down the road though so far it hasn't exploded.
Immigration is a really bad mark for her in my book. She was put in charge of fixing our immigration issues and she has don't nothing from what I've seen. She went to the border once for a speech and photo op, but that doesn't really fix much. I only expect that to get worse when she has all issues of the Oval Office on her plate.
Foreign policy is where she very much concerns me. Obama fumbled through and made mistakes like drawing red lines with invisible ink. He had the fortune of doing that when we were still primarily dealing with terrorist actors globally, the only state actors directly in play were smaller. Today we have Russia still in Ukraine, NATO both moving away from the US and sabre rattling with Russia, and Israel continuing a bombardment if Gaza with the goal of completely annihilating a terrorist organization while simultaneously fending off threats and attacks Lebanon, Yemen, and Iran. Harris bumbling through similar to Obama will be extremely dangerous in my opinion.
That said, we largely agree on Trump's record. I don't understand claims made that he is an immediate threat to democracy or implement some kind of fascist takeover. He is a bad leader though, and a loose cannon which is dangerous given global problems today, albeit a different risk than incompetence.