May be because Phantasy Star Online Episode III: C.A.R.D. Revolution did not sell well so they abandon that kind of gameplay.
Right now the game's gameplay was appreciated and praised for solid and well-thought-out mechanics, but that's not the case when it was released.
Most PSO fans back then expected it still to be action RPG instead of a turn-based card game format.
Personally I love SEGA's trying to do something different but they're not supposedly naming a spin-off game into a sequel (Episode III).
Looking at gameplay footage, I can see why it was (to a degree) disappointment. PSO being an action adventure game, now ARKS resolve their combat with turn-based card system than just taking a sabre or gun to the problem.
Perhaps, Phantasy Star Online Episode III: C.A.R.D. Revolution would've been better off as an
actual card game? I imagine, though, like MushiKing and Dinosaur King, it would've mostly been popular in Japan than the west. But may still have "some" market in the west given how popular PSO was in general.