<< NT4 has had 7 service packs(6 and 6a) since its release, so its not as if they just built it and never added anymore to it. It should have been worked into 6a. >>
The service packs were more about fixing the bugs created by the previous service packs than about providing extra functionality. They rarely added any new stuff, although they sometimes changed or simplified things.
Additionally, it was also a tactic to try and get NT users to upgrade to Win2k. "If you want USB, use Win2k. That is all."