Key thing is to finally get rid of cage and unified UFC rules. Cage is just too big, it was so much easier to avoid Mirko's kicks and left hand in such a big open space and wait for counter, and of course there is too much long, exhausting, teadious wire fence clinching in cage where leaning of dead weight gives advantage.
Of course, huge mileage and injuries played their part, but american UFC version of MMA and unified rules were simply terrible for Cro Cop, and helped much to his opponents. Cro Cop just needed some career sports luck to not have to fight in UFC and cage at all if possible, and at least not till 2008 till he gets rematch with Fedor while his body was still holding.
BTW Pride had far brighter lights than UFC while it existed. Pride had more money, was more watched worldwide with huge stadiums filled, and Pride had better fighters while UFC was B league. So Cro Cop did fine under bright lights and on the biggest stage.