Compensation is a big part of it. I think a wrestling base compensates for weaknesses in other areas better, that's why it is so dominant. A great striker who is weak on the ground can stuff several takedowns, and still lose the round if he gets taken down on the 3rd 4th 5th try and probably won't get back up ever, they can't compensate as well.
A wrestler can have lousy striking and still hurt people with a big shot if the wrestler has a decent chin and can take some shots. Kos for instance, is a god awful striker, but still knocks people out sometimes. Strikers don't suddenly land a 'lucky' takedown against a great wrestler, it's practically unheard of, and even if they did, a superior ground guy would just get back up.