Probably due to a malfunctioning or poorly designed compander circuit. All analog wireless systems will introduce noise (hiss) into the audio, some models do it more than others. Some models of the more expensive brands will have more noise than a cheaper system - and sometimes it's environmental.
Try operating the system with no mic plugged into the transmitter; if you still hear the same level of hiss, it's probably a bad cable, external RF, or a design issue. If you don't hear hiss, then the compander circuit may be the problem (still a design issue, but at least you know it's not a bad cable, etc.) If, when a source is transmitted through the system, the hiss "follows" the signal level, it is probably the compander.