Soylent: I believe one main deciding factor when choosing which version of DirectX to use in a game is the platform(s) on which it will be available.
Since Witcher 2 was going to eventually make it to the Xbox 360, it had to be written with DirectX 9, which is the maximum version the Xbox supports.
Also, in recent years, console games have become much more profitable than PC games, so rather than writing a PC game and porting it to console (or writing three separate versions for Xbox, PS3 and PC), publishers dictate Xbox first, PS3 second, and PC last (if at all).
What about Battlefield 3 then!, :D .
The minimum requirements for the game are DX10 on PC.