A 6" wafer will also have lots of dies on the edge that don't pass automatic testing. Don't forget that most devices are located near the edge.

Secondly, upgrading your Fab to switch your process to larger wafer size has a 10-figure $ price tag. It's not trivial for digital, let alone opto electronics. LEDs are special in that they deviate severely from the standard mos process. When Monsanto first produced them they almost dumped the idea for LEDs because of all the issues to deal with the exotic materials.