Good question :-)...
There are a few reasons (please don't take these points as complaining - simply reasons).
- I had to reinstall my system as I got new SSD drives so I was starting from scratch with a Windows 8 OS.
- FSX has a number of problems with Windows 8, one being very critical to the operation of FSX, my flight controls would disappear mid-flight and attempting to land would become near impossible without AUTOLAND. There are a number of posts for the USB power down but none of these solved the problem (uninstalling Windows 8 and putting Windows 7 is not an option considering that the flight controls are still available in Windows so it definitely is an FSX problem).
- Having to tune FSX all over again, tweak, make config changes, etc. seems like a waste when I read a lot of people with P3D just running it and not having to care about menu crashes and putting a DLL in the main directory.
- Installing ORBX scenery and all other "enhancements" multiple times - waste of space, but also, can you even install on the same OS to two different locations... I don't think all programs support this.
- Paying for new aircraft when I know P3D is beginning to get some traction with the sim community... it feels like spending money in FSX is a waste... unless I could be guaranteed that the purchases will work in P3D with no further monetary investment.
My main issue is really regarding the disconnection of my flight controls which really ruins my flight. Unfortunately the only way to fix it is to Save / Shutdown and Restart FSX. This is not an option as my Virtual Airline will not allow a flight that has been saved / reloaded to count :-(.
If only life was simple :-). After reading a thread provided above I can see that PMDG is attempting to license P3D for their aircraft so I am hoping that we are simply talking about a few months at most before we see their aircraft in P3D...