Why?
Because he is a bully. He puts people down, bad mouths them, smears them, attacks their past, their career etc.
Just some recent examples
Judges
Senator Mcain
Senator Blumenthal
Hillary Clinton
Meryl Streep
The NY Times
CNN
I am sure people can add more - it's his tactic. Can you imagine being married working for him?
He gaslights. And repeats it.
Note how he also praises people - good guy, great guy
How he uses sales patter and 'agreement ' words.
He slams people who don't agree with him and shames them publicly.
I am sure MN has terms for a person like that.