Honestly, it's the combination of large, unaddressed problems (healer balancing, role stacking, heavy melee disadvantage), repeatedly enforced bad strategy ("Mid = win", trying to leave towers up, Brutes being wasted on fighting/blocking players and not attacking/defending objectives), and poor sportsmanship (loudest complaint is usually the biggest quitter, people giving up soon as they fail to cap mid/lose a tower right away) that drive people away from the mode.

I like Rival Wings, I really do. But as a veteran PvPer, as someone willing to keep fighting, even when things are looking bad, and as someone with the mount who's still willing to queue, I don't want to play if that's what I have to deal with. And if SE's seriously looking into possibly restricting pre-formed parties, in the sense that I won't be free team up with my friends that I KNOW I can count on to use good strategy and not roll over for the other team once they think they can't win, I'll have even less desire to play.