To me, there are much perfect AKB member for JAV
Just because
you would like to see them in JAV doesn't actually mean they are "more perfect" in any way. Both Erena and Haruna are far, far more popular in AKB than Rina ever was -- and it's popularity that decides who stays and who graduates.
It's also popularity that influences what they can do after leaving or graduating from AKB (or any other pop idol fame). With as large a fan following and as much on-TV/radio experience as they have, they have far more opportunities to continue a career in entertainment before having to ever consider JAV. Erena herself has left AKB to study abroad in the hopes of becoming a true actress -- something that would be impossible for less popular idols.
Seiyuu is unlikely for an ex-idol. They simply don't have proper voice training, and their professional experience has been about developing their own idol personality; they have no experience developing the personality of an anime character.
Indie labels don't want generic pop stars. Rina would have to show true musical talent (and not just "I like singing and dancing"); but if she had any of that, she probably would've been picked up by an indie label before joining AKB.
And being an independent musician is even harder. Frankly, AKB members (and most pop idol girls) don't earn all that much money. Enough to survive in Tokyo with a little extra to play once in a while, but not enough to secure a career. And composing by herself is a joke; she'd have to go to school and take classes on music composition, considering how all of that was done
for her when she was in AKB.
Although we will never know the
real reason why Rina went JAV, it most likely has to do with how, when she moved to Tokyo to be in AKB, she often felt lonely and therefore was impressionable to an extent. And how she seemed to really enjoy the popularity and living in Tokyo, and probably had some aversion to giving it up and returning home. And by her own admission of losing her virginity to a random AKB staff, her comfort with her own sexuality.
Since Rina really wasn't popular enough to get other jobs in entertainment, JAV probably seemed like an easy way for her to keep her pop idol lifestyle. And to be honest, she's far more popular
now after doing JAV than she was when she was in AKB. But only because she made the jump, not because she has shown any potential of becoming a professional entertainer post-AKB.