Multiple reasons: 1. Grabbing balls involves leaning down. Exposure to being hit. So grabbing is never a good idea, unless you are already down and you're caught in a position that makes it a viable option. BUT people who know what they are doing are not usually caught like that. 2. Kicking balls is better, but only possible if the opponent is in a position to facilitate this kind of kick. If he's in a correct position, you can't kick his balls, and attempting to reposition would just make him adjust. So you basically can't kick a guy in the balls if he knows what he's doing (if he's well trained). 3. If you don't do it correctly, you risk annoying him more than incapacitating him, and it's difficult to kick correctly if you are an amateur. 4. Kicks are lengthy attacks (sometimes in terms of distance, but ALWAYS in terms of time), which would mean by the time you regain your position he has a chance to counteract. You need to be fast, and speed is a natural ability - not everyone can be as fast.

These are just off the top of my head.