If your just talking Wrestling and Boxing then I see no reason why either cant learn the other discipline. In MMA there are so many other disciplines to learn you cant devote enough time to any single one to be world class. In the case of a Karate fighter it's in the stance ( not squared up ) and there is always the fear of the TD so it may not be in your best interest to dig in. I think it's more to do with not choosing to use the correct technique rather than not being able to.
haha I totally believe that...well the getting hit part is from experience. I can do all the kickboxing I want but unless I get hit don't mean much
same as BJJ you can drill and roll (70% max) all you want, but if no one is going for subs or all out you won't progress
NO ONE likes getting hit HARD