The most obvious reason is ice and wheelchairs are not totally compatible. Simply think what would happen, if the NHL would play on a sheet of ice as large as a soccer field. The speed and intensity would not be the same. As with any sport, uniformity of equipment is important. A plastic ball is substituted for a puck, allowing greater movement during the game.
Many players simply use the power of their wheelchair to move and control the ball.
Protective equipment, such as helmet, pads and eye protection is optional at this point. Most contact during the sport is primarily from wheelchairs bumping each other. Of course, rules are adapted to fit the ability of power wheelchair users. Sports opportunities for power wheelchair users are VERY limited. In fact, until recently there were NO power wheelchair team sports opportunities available.