Exactly. PS/2 is a wire protocol which was made for keyboards: it sends only 3 things over the wire:
-command
-key press event
-key release event
The only limit here is in buffer size (16 events) because your computer may not always be able to process the events in a timely manner.
USB on the other hand is a general purpose serial wire protocol used to connect peripherals to your PC. Different devices will use a different protocol on top of USB, for keyboards it is USB HID (Human Interface Device).
USD HID doesn't use key events, instead it uses a special kind of "data packets". Each time the keyboard is polled, it sends a table of keys which is currently held down. The computer can then figure out which keys are pressed or released by making the difference between the previous table and the current one. The limit here is the table size, but most devices are able to choose their own size. The only exception are keyboards.
When you turn on your PC, you may want to get into your BIOS setup. For this reason, USB keyboards are required to use a simplified and fixed way of talking to the PC so the BIOS doesn't need to know everything about the USB HID standard. Instead, the key table is limitted to 6 keys + modifiers.
Custom USB drivers could be made to understand a custom HID "data packet" so they contain more keys. It isn't so feasible to do though as you'll need to make one for every flavour of OS. Because of this, most vendors simply tend to use the default drivers instead and we have to live with this limitation.
I suspect there might still be a way around all this, at the risk of incompatibility with some operating systems.
