Because:
The EMP grenade is a serious contender for the most devestating grenade. Though it has no explosive power of its own, it detonates anything explosive you are carrying. Thus, if you are a soldier, it will detonate all the rockets and explosive shells, ie shotgun shells, you are carrying. This basically makes it devestating against most classes, as even the lightly armed scout still carries enough explosives to make sure you can take them out. The only effective defence against this grenade is to make sure that you have discharged all your ammunition - you will basically become invulnerable to this grenade
