I think it is a question of three moments.
1) Availability. Linux driver stack is unfinished, unpolished and lacking lots of functionality - if to be compared with catalyst(yes, big "lol") or nvblob.
2) Lacking manpower. Linux driver stack itself receives 1/100 of attention it "should".
3) Licensing. BSD is steal-friendly. Where GPL would give good protection against raw stealing(ie. concealing), BSD is plainly give-away. I doubt any company would share any IP unless it is well-known by others and hence outdated, cause that would mean simply loosing advantage vs minor gains(bsd moneyflow).
Panix points are valid in many cases.