Same as VGA, shit just works, don’t need to worry about drivers or OS. So if your server shits the bed, you don’t need to worry about these things not working so you can can figure what went wrong.
I'm not completely sure but I'm pretty sure that at least the majority of our servers only have USB and no PS/2 port. And while our servers aren't very old some of them still have a few years on them.
I don’t know what server you’re running, but I have never had any issues with USB keyboards. They just work, including in the firmware. No drivers needed. Besides, a proper server motherboard will have IPMI so you can just remote into it.
I've had KVMs that don't like the 'fancier' USB keyboards with NKRO. It would work, but it wouldn't listen for its own 'switch to different console' hotkeys. Reconfiguring the keyboard to run in 6KRO-only mode addressed it, but not every keyboard can be configured that way.
You've just gotta dig out the mouse/keyboard combo that came with the Packard Bell you bought in 1996 from some dusty box in the attic / storage room at work