ATI and NVidia are mainly graphics cards companies. They don't have anything other companies are buying usage licenses to (well, NVidia has PhysX but I rarely see that), so they have to make a ton of cards to stay in business.
Microsoft makes DirectX. And Windows. And Office. And. And. And. anyway, from the way I understand it (which is likely wrong), ATI and NVidia have to pay Microsoft a tiny bit to be able to put DirectX whatever in their cards. Even if that's not the case, people are still buying a license to Windows all the time. So Microsoft can afford to take it easy and only push out a version of DirectX every three years.