Ice Breaker probably got left behind for the exact same reason Black Hammer's ammo regen was nerfed to take from reserves. Both weapons made nearly every boss fight a breeze. The only reason Black Hammer got an exotic upgrade and brought into Y2 was probably because it's ammo regen was reworked to take from reserves and it required some actual skill to get it to proc (except on Spider Tanks of course). Ice Breaker got left behind probably its exotic perk guaranteed you never ran out of ammo for it or any other sniper really. It's a shame because both guns were actually the only true exotic weapons in my opinion... Well them and of course Gjallarhorn.