Legacy (the development started in 90s), a lot of different CPUs/SoCs to cover (from various vendors, none of which are famous for their mainline-ness), drivers

Legacy (the development started in 90s), a lot of different CPUs/SoCs to cover (from various vendors, none of which are famous for their mainline-ness), drivers for ICs that are not publicly available (that you're forbidden to distribute in source form). Not that other networking companies are better in this regard, they just have been founded when OpenWRT was already around.