We have 6 Phoenix characters, but only one leadership. I suspect Hera's lead is so good that a second lead would rarely get used… except for 3v3 GAC. It makes sense for there to be more than one Phoenix lead, since the other characters often took turns leading missions in the TV show. Of course, giving Ahsoka Fulcrum a Phoenix tag and leadership would solve this problem, but that dead horse has been beaten beyond all recognition by now and it is clear the Devs are never going to give her the tag (despite the fact that Phoenix was HER squadron even before the Ghost crew joined them).
I realize people are going to say to run the others under rebel and/or Jedi leads, which is what I do. But most of the zetas for Phoenix are specific to their faction, so you lose a lot by putting them into other squads.