I can imagine many developers are in any hurry to develop a game that cannot be run on 98% of systems. I'm sure even 3yrs from now almost every game will be able to run in some form on DX8 compliant graphics cards.
Looked at the much hyped UT2003 for example, drop the details and resolution far enough and even pre-DX6 graphics hardware can run it passably well, and anything beyond that can run it quite smoothly if your willing to sacrifice visual quality enough.
There are still only a an extremely small number of games that require a fixed function HW T&L capable graphics board, and those that do can all be forced to run via software emulation on earlier hardware via simple software hacks.
Developing a game that absolutely cannot run in any form on non DX9 graphics hardware would be foolish at this point in time, your immediately cutting off virtually the entire market and ensuring that even much of the already extremely miniscule enthusiast market cannot run it.
It's a perfect way to ensure your game sees next to no market success and is a complete failure financially.