Subforums are also places where active threads to go die. Much fewer users look to subforums than the main one, as such, while the concentration of information may be higher, the distribution of said information will be far lower.
Added to that that the FP+ frenzy is likely a temporary thing. Once the system is up and running and the dust settles, the threads will switch back to focusing on FP(+ or not) strategies, which have lived in this forum for at least the ~4 years that I've been hanging around. We didn't have a subforum for normal FP so why would one for FP+ make sense? We didn't have one for New Fantasyland when these forums were full of questions and information (and misinformation) about the expansion, and it's since died down.
So, to me, it simply doesn't make sense to pigeonhole them as they are as much (if not more) about attraction strategy and information than most of the threads here.
However, I'm not a mod here, so my opinion means little for this and even less on how message boards in general get run*

. They'll do what they think is best for the boards and FP+ is on their radar (they mentioned it in the last thread).
(As a special note, for all those saying that a subforum would make it easier since all the threads would be in the same place... aren't they in the same place if they're all in the root TPAS forum?

)
(*Note: Not a slam or critism of the mods in any way shape or form! I know it sounds like it could be one of those griping things that people say "Pfft, I'm not a mod so my opinion means nothing!", but it's absolutely NOT said like that. It's just that my opinion in general shouldn't affect how the mods themselves (each with their own opinions) run the boards to the best of their abilities. I tried to make it sound better a few times, but I gave up and created this overly wordy disclaimer instead.)