For most of the off-season, the Sox looked like the bullpen would have Jansen, Martin, Winckowski, Schreiber, Bernardino, and one of Houck or Whitlock. Why would anyone put Campbell third?
The trade of Schreiber and the injury to Giolito dug into this depth, and the inexplicable demotion of Bernardino didn't help. But none of this still ever made any of the arm you mentioned as the third best option in the bullpen.
And two weeks into the season, both Slaten and Weissert look better, as well...