Because its a skilled power weapon, not very noob friendly. As is the sticky detonator, concussion rifle, and the Reach grenade launcher. Most people suck with it, but in the right hands and can be deadly.
While weaker than the Spartan Laser, it charges quicker and you can hold a full charge for an extra second instead of being forced to fire immediately at full charge, making it more reliable in a pinch. Plus it just seems cool.