Because that is all everyone will use for specials in PvP after the totally unnecessary special ammo nerf.
Bungie is digging their own grave. D2 will be a well-deserved flop if they continue down this road.
Edit: Since a few people seem to think I am whining about this, know that I barely play Crucible and when I do, I use primaries and sidearms. I am simply stating the proven fact that when Bungie nerfs something, something else replaces the nerfed guns as the new meta, and then the community will start complaining about [i]that[/i].
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Sidearm meta will be fine just like the original auto rifle meta was. Side arms will reward players with good aim who hit more headshots consistently than others just like the first auto meta did. Those who can aim will win the gun fights. Those who can't will call for nerfs. One thing we can all finally agree on now. No more nerfs. Destiny really can't take any more nerfs.