They haven't settled after the new CPU tech, yet. They are just a bit messy, and there is only one that has three usable PCI-e slots, for example (and the DirectX10 cards, for example, may be much better use for PhysX calculations, as it allows the calculations that are output by the GPU to be used as input).
The mobos at the moment are just too "patched" ... even the Intel-compatible ones haven't all standardized on the new standards (details escape me at the moment, the 975X chipset thinggie (Northbridge interface?): some mobos use the 965X, and are faster).