Because NLB is actually the worst on that list. Pretty much everything it does a normal sniper can do better, doesn't take up an exotic slot, AND doesn't give you the ammo efficiency of a hose with 18 holes.
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Exotic primaries are much better for crucible, and SGA is replaceable with nearly any legendary, and definitely other exotic heavies. Just not practical, ever. Unless you really have zero good weapons for oracles, in that case learn to use relic.