Considering people were saying it would be impossible to make Rise a fighter, and now she's in, I think it's kinda short sighted to say that it's impossible for Fuuka to be made into a fighter. I think ASW are creative enough to pull something off. Plus, ASW originally wanted to make Dojima/Nanako a fighter. Think about how the hell that would work at all, yet ASW still wanted to do it.
But yeah, her popularity compared to Rise is the main barrier of entry. She's one of my favorite SEES characters, but I feel I'm one of the few Fuuka fans out there. That and they don't really have a reason to do it. We have enough P3 characters, in a game titled Persona 4 Arena, as it is. Can't have the P3 cast outnumber the P4 cast (one of the reasons why I think Rise became playable).
It has never been anything but stupid to say Rise couldn't work. They always had the idol motif. Rise is leagues more popular, sure, but it's the former that puts the kibosh on a Fuuka, not the latter. The designers have nothing to play with.
The idol thing is an aspect of Rise's character they can embellish and they seem to have made good use of it. With Fuuka they'd need to pull something out of thin-air.
I love Fuuka, as you all should know.