Mourinho (willingly) created enemies the entire time he was there. He then used those enemies to always have an excuse when things didn't go his way (which happened A LOT at Roma). But it's what he does everywhere he goes. Creates an "us vs the world" environment. Roma fans are so starved for any success, they actually love the guy. Even when he feeds them shit left and right, their team played shit football, got shit results, etc....they still backed him. He was smart to be on the side of the fans with the whole Bonucci thing just recently also. The guy does everything in a premeditated manner.
But their goal has always been top 4. They didn't hire him and pay him a shit ton to finish outside of the top 4 every year. He failed at that. And the Friedkins probably got tired of him bitching about how his squad is of low quality and how he lacks options, blah blah. Also, the fiascoes with the referees became a monthly special. It became embarrassing.
I do think he had a certain pull with players on the market, for example Dybala and Lukaku. But at the end of the day, he plays antiquated football and his antics and mental tactic bullshit no longer works. Everybody can smell the rat's bullshit from a mile away (except Roma fans, apparently).