Black hammer does more damage than icebreaker, an exotic sniper. And in its current state, it has unlimited ammo that "regenerates" faster than icebreaker. On top of both of that, it does more damage to hive majors. And on top of that, it's a legendary, so you don't have to waste an exotic slot. Basically, it's icebreaker's little brother who superseded it in every way. I'm not defending Bungie's decision to nerf it, but simply providing hypothesizes on why it was nerf'd, such is what you asked for.