NT Server got as far as version 4 (or 5 if you want to include Windows 2000) but is now obsolete. When it was first developed in 1993 or so there was no need to support USB as that was not a final product at the time. Of course, in its later incarnations (W2K, 2003 Server, XP) it does indeed support USB.
Some call NT Server the OS that refuses to die, because it is robust, fit for its purpose, not overloaded with bells and whistles, and most important of all, given all that, cheap. Why change to the vigour and rigour of a 2003 Server environment, and the cash haemorrhage that is Microsoft licencing, when the trusty NT struts its stuff day in and day out without complaint and is there already?
During the 1990s NT was developed as a separate design and by a different team from the domestic OS that became Windows 95 (No USB support here either). There was a bit of cross-talk, but early on the 95 people ditched a central idea of isolating the so called "kernel", the absolute core of the operating system, from the applications that are run. This is why 95 and its successors 98 and ME are inherently unstable, whilst NT works like a dream. The 95 user interface (click on the"Start" buton to stop etc.) was adapted to NT but that should not lead you to believe that there is anything similar "under the bonnet"