Think there are a few reasons.
1st is these companies don't have a clue on how to write drivers. They use nvidia drivers for nvidia cards, and motherboard bioses are the most major thing some have to write.
2nd, ATI drivers are far harder to write then nvidia. They have so many features, with dvd, dual monitor, truform, smoothvision, etc. The GF cards really only have their AA to deal with, the GF4s have a bit more now, but still not as much as ATI.