This is a topic that has been bantered back and forth for some time now. The answer as to why P3D topics end up in the FSX Forum is quite simple. . .the solution to keeping them where they belong is not.
The Answer: We are creatures of habit. Until the release of P3D we were either here or in the FS9 Forums. Those two have a very dedicated and active following and the Sims have remained separate for the most part. When P3D arrived it was viewed initially with much caution based on who could use it (and we will not venture there for obvious reasons), suffice to say as the "new kid on the block" it started out slow and folks wanted to see feedback from users before taking the leap, keeping one foot in P3D and the other planted firmly in FSX where they were comfortable.
Over time, and with the familiarity everyone had with FSX and all it's quirks, while they would post a question in the P3D Forums, responses were few and far between and so. . .frustrated with the lack of assistance they came back to where "everybody knows everybody" and where responses came quickly and problems were solved by the amazing brain-trust that resides in this forum. P3D users are not here to bring down the FSX forums, many of us have both Sims still installed and despite the fact that P3D has it's own Forums, unless it becomes more active you will continue to see questions posted that pertain to that Sim.
The Solution: Simple, there is none, as the saying goes. . ."You can lead a horse to water, but you can't make em' drink". If users of P3D aren't getting the info or answers or assistance they need on the dedicated Forum, then they will return to their roots.