There are only two tasks harder than writing Free Software graphics drivers. One is running a successful crocodile petting zoo, the other is wireless bungee jumping.
In general writing graphics drivers is hard. The number of people who can actually do it is very small and the ones who can do it well are usually doing it full-time already. Unless the company, which those folks are working for, supports open drivers, the earliest someone can start working on open drivers is the day the hardware is officially available. That's already about 2 years too late, maybe a year if the hardware is just an incremental update. Obviously not a lot of programmers have the motivation to do that. Small subset of the already very small subset of programmers who can write drivers. Each of them worth their weight in gold (double, since they're usually pretty skinny).
Vendors who ship hardware with GNU/Linux don't see a lot of value in having open graphics drivers on those platforms. All the Android phones are a good example of that.