The difference is that NLB takes primary ammo and one shots to the head. While NLB takes skill to properly get kills with, the fact that it would ignore the ammo economy in Crucible could potentially lead to mass abuse. Sure, it could just take Special ammo, but then it'd just be a gimmicky weapon at that point. Weapons like NLB, Universal Remote, Invective, and Icebreaker were all removed in D2 due to their ability to ignore the ammo economy. Whisper was nerfed to preserve that a year later, although, this was Bungie's fault entirely for bringing back a weapon in its unnerfed state.