Mainly down to the image quality Paul. You can only have AA set within FSX, which only gives you 2x AA I think Compared to the settings I use in DX9 it just doesn't look as good.
Also, I actually get slightly better frames rates in DX9
Still, that DX10 flicker fix seems to be a good 'un
DX10 is OK but it has it's faults. Main one is lost Nav lights which is well documented and for me the VC cockpit shading idea is great especially when the sun shines through any framing with the shadows and would be very realistic if the edges of the shadows were not saw-toothed edges.
As it says on the selection panel in FSX 'DX10 Preview' which suggests that it is perhaps a beta sample of what was intended to come but sadly never did.