I think the biggest thing with having support for both DX9 and DX8 would be that, Scawen would have to maintain two versions of the graphics engine and it could prove to be a big time consuming task, because DX9 and DX8 are quite different. So in LFS case I think it's best just to stick with supporting one version of DirectX.
Moving on to DX9 (or even DX10) would be a good addition, because they are better than DX8. There is just one thing blocking the move to newer DirectX: The users who are still using old GPU which don't have support for newer versions. Even Valve insists on maintaining support for DX8 on Team Fortress 2, because there is a lot of people still using old graphics cards. And I think the userbase of TF2 is more eager to upgrade their computers than the userbase of LFS.
All in all, we are probably going to stay with DX8 for a loooong time.