We have to look back to understand why is there a DX9 support.
At the time of release Win7 was not wide adapted by the users, Vista ... well ... was Hasta la Vista
, so there was a wide base users with XP.
Madoxx attended this users claim and released the game with DX9 support stamped in the box.
The real problem now is that you can´t go back and release a official patch now without DX9 support because the Steam Auto Update will break the game for those who are using the game in XP, meaning are playing in DX9.