Many graphics card manufacturers have not yet published updated drivers for Windows 8. Windows 8 comes with default drivers written by Microsoft that will make most GPUs work, but only enough to run Windows itself. These drivers are not made by the GPU manufacturer, so they don't use the hardware very well and provide only the most basic graphical functions to software that wants to use the GPU. Minecraft can't run on a computer with these default drivers.
So the short answer is, for some GPUs there are no Windows 8–compatible drivers that will run Minecraft. Not yet. And if it's a niche GPU, then maybe never.