You practically have to stick the barrel down something's throat to be a threat to it. And while you're trying to do that, you're gonna get clobbered by any enemy that matters. Meanwhile, snipers wreck stuff, absolutely wreck stuff from med-long range, and fusion do the same from short-mid range. There's no "niche" that a shotgun fills that Snipes/Fusions don't do 10x better, while allowing the user to stay out of melee range of the target.