x10 is no longer officially being improved or worked on by TGC.
x9 is being improved and worked on rapidly by TGC and many other members of the community.
Keep in mind that just because directx 10 and 11 are newer than directx 9, it doesn't really say anything about the popularity of each. X9 is still the primary version that is being used by many engines and final commercial games do to its tried and true stable state and supported hardware.
I look at it the same as comparing webkit based browsers as apposed to IE. Webkit normally supports the newest tech IN DEVELOPMENT where as IE only supports the new tech that has been tried and trued through testing.