They do not feel like they should take up an exotic slot; and their jobs can easily be done by many other things in the game.
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Imagine having to get 6 kills to charge that weak -blam!- hunter glaive just to fire a lackluster tiny spark that zaps a few adds when you have wave frame grenade launchers. I mean, they tested these and somehow have to KNOW they´re not exciting at all, right? The hunter glaive can´t do a fraction of the work trinity ghoul does, and that one has unlimited ammo and is ready to go after ONE kill.