I've never wrestled, but from watching it, I'm going to give this one a guess. The chances of you getting hit in the junk are actually a lot smaller than having your package sandwiched to the mat. Wearing a cup not only would be quite cumbersome, but would put an extreme amount of pressure -- and pain -- on the outlining regions of the cup. When a wrestler is riding someone into the mat for three minutes (just an estimation on a regular, seven-minute match), that area is going to become very, very sore, and thus, quite painful.
That's just a guess, though.
I got hit in the cup during baseball a few times, and each time it ****ing hurt. The nuts were OK, but that area stung for quite a while.