From the FlightBeam website-
Why isn't KPDX available for FSX and/or P3Dv3?
Prepar3D version 4 is a very different from either previous versions of P3D or FSX in one major respect - it's a 64-bit simulator. That means the sim can use all the RAM your system has rather than a small, set amount. The plus side of this is that out-of-memory (OOM) errors are a thing of the past. That gives us the room to add detail like we never could before without killing performance. By authoring for this new environment, however, we're using techniques that don't translate well to the older 32-bit sims. Creating a version of the scenery for older simulators therefore takes a lot more work, and this is at a time when our research shows that use of those sims is rapidly falling off. So the short answer is, it's not worth the amount of extra effort that would go into downgrading the scenery for 32-bit.