Is it not a standard for USB 2.0 across each OS to register 6 keys pressed at a time? My issue with USB 2.0 is THAT standard. If a company made drivers for their peripheral for every OS I guess there would be no trouble, but by standard PS/2 was able to do NKRO regardless of the OS so why was that not included the USB standard?
No. The USB standards don't say anything specific at all about keyboards or mice etc., in any version.
It's the HID standard that does. The 6-key plus modifiers report structure was only really intended to make life easy for BIOS (so they didn't have to parse report descriptors), but became a de-facto 'standard' for keyboards regardless. However, the HID standard allows for all sorts of different reports, and all major OS support it fully, so there's no need for extra drivers.
The other limitation is that a low-speed USB data packet can only be a maximum of 8 bytes. A report can be larger, but that starts taking a significant amount of time to send. The USB standard only allows a low-speed device to send one packet every 10ms (per endpoint). OS typically nudges that up to 8ms, i.e. 125Hz (without hacks). Taking up to 16ms to get a keypress to the host would be crap! So we've had a phase of makers trying to get more out of low-speed chips than they can really handle, with tricks like multiple endpoints etc.
Full-speed USB frees up those limitations. Each packet can be 64 bytes, and can be sent every 1ms (again, per endpoint). This allows the HID standard to be used to its full effect.
Finally, NKRO isn't done just for the sake of it. Doing full NKRO requires only one bit
per key, rather than one byte
, and doesn't have to treat modifiers as a special case. So a keyboard can send e.g. 104 keys in a mere 13 bytes - using the other scheme 13 bytes would only manage 11KRO (11 keys + modifiers + one reserved byte). Sure, that's still plenty of keys, but it's far less elegant a method (and still 'not-6KRO', so could still be incompatible with dumb hosts).