I can imagine lots and lots of reasons why a MotoGp helmet is desgined that way. Aerodynamics and multiple layers of protection being the main reason. But a motocross helmet has its front pointed. And a sunshade(it seems). How aerodynamic is that anyways? In case if the rider should fall on his/her nose, the long pointed end of the helmet may prove to be counter-productive and it might hurt/break the neck. But i can guess why do they insist on using goggles instead of visors. Is that the reason? That a person can use goggles which fit perfectly inside the MotoCross Helmet?
It's all about safety for the specific requirements of motocross.
In motocross you do not want the weight of a full helmet when you are coming down off a jump, so the lighter the helmet, the less strain on your neck.
- The peak is designed to snap off in the event of a crash, so that is not a problem.
- Aerodynamics are also not an issue - motocross speeds are relatively low.