USB 3 is significantly more complicated to implement, and the hub chips are quite a bit more expensive. Hardware-wise it would've become by far the hardest part of this board.

USB 2, on the other hand, is fairly trivial. You almost have to try to get it wrong - especially when you are not concerned about certification.

Going by the task at hand, registering keystrokes pressed by a human, USB 3 is also not quite needed.

PS/2 is far more trivial and low-latency than USB.

I'll remember that next time I'm developing an FPGA keyboard for my Gateway computer.

Many of the latest gaming mobos still have PS/2 exactly because of the low latency.