Eh, I don't feel the game's popularity's an issue. At times it seems like every game, not just the most sold or even most acclaimed games of the past, gets re-released or re-re-released, just because it's relatively cheap to do.
Whatever the reason Sega hasn't made another PSO Card game, I'm sure that Episode III will return one day. Maybe on consoles, maybe on a portable device and maybe even just as an iOS game.
I feel I'm not ranting enough to post in the Rant forum, but I can't help it: I'm a nice guy!
Also, I love Episode III, it's the best game ever!!