Mirko is a living legend and despites the fact that he never captured a real Pride or UFC belt he will still be remembered as a true pioneer and legend of the sport, way before some real champs.
He was a top K1 fighter during an era which I would consider the best or at least second best time in K1, he later went on and won the last official k1 tournament ever (don't forget the first tournament was won by Branko Cikatic who was Mirkos coach at the very beginning of his career), he was a top2 fighter in pride and won the last OWGP, and he went out on a 10 fight winning streak.
His UFC run was disappointing for all of us but in retrospective I am not shocked he did not bloom in this new environment where he didn't feel comfortable at all, and I think I know the reason for that.
I've followed Mirko for a long time, inside and outside the ring/octagon, and without any hate or negative feelings I can say that Mirko has some tendencies which would make me believe that he has some slight autism. Again, this is no hate but just the summary of the observations I have had over the last 2 decades. Autism doesn't mean he is stupid, no vice versa actually, Mirko is partially a very intelligent man and you can discuss some high brain topics with him if he is into them. But that's how most people with slight autism are...
I have rarely seen a person which is so much into repetition and especially a routine life as Mirko, which shows not only in his training where from a young age he would train single punches and kicks over thousands and thousands of time without getting bored by them, but also in his daily routine which basically didn't change since he was a teenager. Its enough to say that he hasn't changed his nutrition and eating habits since he was a kid, he only eats a small portfolio of things and he eats that every fucking day since like 40 years, and if he is forced to eat something different (as for example in hotels before fights) he is pissed and hard to deal with (as also trainings partners confirmed who travelled with him). The same goes for his family vacation, the guy, despites living in a beautiful country with one of the best coasts in Europe, only travels to 2 destinations, which of course, he visits since he is a kid. Slavonia where he was born and Pula in Istria, where they used to go for vacation with his parents. He never goes anywhere else, really never, not even take his kids to typical destinations which parents with his financial situation would do 100%.
Moreover, his unwillingness to train outside of the comfort zone of his own backyard seems more like a xenophobia from other places than just pure comfort. And thats also the point why he is his own coach! Coaches challenge their students and try to bring in new stuff, Mirko doesn't like to do stuff he didn't plan with and he wants to know what he will do from start to beginning, this is why he plans his training. As mentioned in the paragraph above he never travels anywhere unknown and if he has to he will only stay the bare minimum time there, this is why he never arrived earlier to an Event and tried to acclimate or deal with a jetlag, he just accepted it. Japan became a small exception as he traveled there so many times so sometimes he would stay one, two, three days longer and train in a gym of somebody he knew, but all that is far away from training camps we know from other people. As another example, for his Gonzaga rematch in Poland they travelled by Bus from Croatia to Poland, like 9h, and immediately after the fight went back. He said its much more comfortable to travel that way because he is in a known surrounding, a car, and with his friends, instead of taking a 1h flight from Zagreb to Warszaw where he would need to enter an airport.
Basically what I wanted to say is that I think he couldn't bloom in the UFC because he didn't feel comfortable in there. He said it so many times that he just doesn't feel well in the UFC and I believe him. Mirko never liked change and once he felt uncomfortable he wasn't able to perform on 100%. You could see he didn't want to be in there, although his training performance was better than it used to be.
Long live Mirko CroCop Filipovic