morpha said it pretty much. Games can be made to support all versions of dx, but you need two versions of the engine. One engine that is made with dx7/8/9 and another one that is made with dx10/11. Dx10 and up is not compatible to dx9 and earlier.
There are still many people with older hardware and OSes (mainly people with XP) that are in games target demography which is why almost all games go the extra distance and make the game have both dx9 and dx10 engine. There is currently only one game I know that only has Dx10 engine (Shattered Horizon).
With LFS' in mind having two separate engines would be less than optimal and would increase the already long development times.