For the same reason why, say, version 7 of the WRT-54G router had half the memory that version 2 had. It's expensive (money or calories) and the extra function

For the same reason why, say, version 7 of the WRT-54G router had half the memory that version 2 had. It's expensive (money or calories) and the extra functionality doesn't help in getting more out there (selling or reproduction).