In its' simplest form, the answer is, proprietary drivers and hardware specs. This means that it is a hit and miss operation for open source developers to try to support these hardware devices.
There are two major reasons for this situation. The wireless and graphics card markets are both very similar is this respect. Both markets are very competitive and manufactures are afraid that exposing their drivers and hardware specs will give their competitors an advantage.
Of at least equal importance is Microsoft. They have a tremendous amount of market clout. They have in the past and continue to put a lot of pressure on hardware manufactures not to support or aid Linux. Remember, not only are they a convicted monopolist with a long history of less than ethical business practices, but a simple addition on the Windows up-date site could put any given hardware manufacture out of business over night.
Now do you get it?
Fred