And they are all working on desktop\Laptop support right? Linux is great in the data center because it has big guns behind it in the data center. Linux runs well on cell phones because Google put in the effort. As soon as someone is willing and able to put in the effort on desktop Linux, it will be as good as it is in those other areas.
I feel like most vendors (NVIDIA/ATI/Wacom/whatnot) concentrate much more of their effort into supporting Windows and even OSX becasuse thay's their audience.
Also, I remember reading somewhere that NVIDIA/ATI work closely together with Microsoft because od Direct X[citation needed, though]. I had the opportunity to work with Direct X (the new API) and I found it much more pleasant than working with OpenGL (even though I ended up using OGL in the end; I used Windows and DX to simplify prototyping, because doing the same thing in OGL required much more dev time, at first atleast).
EDIT: Also, let's not forget how most of the majority of linux developer community neglects GUI and the overall end-user friendliness, and how the environment is in most cases quite hostile towards UX/GUI designers in general. There are, of course, exceptions, but those are few.