Scarlet Witch had just gone nut, attacked the whole team, and Magneto had to take her. Thor was in the middle of a Ragnarok and nobody could find him. Carol (still Warbird) had just been court-martialed for killing someone; she was acquited, but may not be willing to test her luck again (it was a big blow to her when she was kicked out of the team for her alcohol problems, and surely don't want to repeat the experience).