The cage allows wrestlers to use it as a wall and has the disadvantage for strikers at least stalkers that they can't back a guy into the corner cause they can always circle out. Counter punchers again have the advantage in the octagon in that they can circle out, they always have some place to move when the aggressor comes ahead. Cro Cop was and still is a stalker and the octagon was never suited for him well because of that. Judo (to a lesser degree Sambo as its judo based) guys are another group at a disadvantage in the cage becasue some trips and throws are just harder to get off in a cage with his opponents back to it, Ropes don't offer the same structure.
Cro Cop's entire game was to stalk his opponents and back them into a corner using a very rangey right cross and heavy kicks. Then all of a sudden there's nowhere to go and Cro Cop can land some heavy, heavy shots on you.
When he moved to the octagon he was no longer able to cut his opponents off and back them into a corner.
Unfortunately he was never able to adapt to the octagon and he lost to a bunch of guys that he shouldn't.
His recent fights in the ring are notable because he's been standing with very high level kickboxers, guys that would destroy Gonzaga in 2 minutes.