I honestly dont know, it is the way it has allways been. Its probobly removed because its unrealistic for someone to run around with a bomb armor. Also its properties might be unrealistic. If you have ever played COD Modern Warfare you know there is two modes ordinary and hardcore. In ordinary u have to empty a mag to get a kill (unrealistic) in hardcore 1-3 shoots kill you, and skilled players prefer hardcore because it is more realistic.
Basically heavyarmor like pepperballgun have properties in SWAT 4 that are not realistic or ruins balance between weapons/protection.
I might be wrong, if so, please correct me!