Why though? It has to be because of his time in Spain. Such a confident and charismatic guy, won everything there is, yet seems so angry all the time. Perhaps he's such a perfectionist that he can't accept some things that happened at Real.
He was never going to be treated like a God at RM but failed to accept this. At Porto, Chelsea and Inter he achieved “wonders” - won them the league or the CL after decades or even half a century. Plus his home record and win percentage was phenomenal. He believed his own hype and thought things would always be like this.
Then he goes to RM - who weren’t successful in the CL for a few years - but had still won enough CLs 7-12 years before Mourinho joined them.
In addition he was joining the country which were reining World and European champions. Those players were the stars Spain at that time, not Mourinho. But he tried his old tricks to divide RM and Barca players and all that and it didn’t work.
Then there was also his style of play which was not appreciated at RM but they knew what they were getting before hiring him.
Basically he was a good manager at RM, nothing more, nothing less. And since then the hype around him got less and less.