The ENT uniforms, while very practical, suffered from the same 'too dress-down' feeling that the 'jumpsuits' from DS9 onward did, IMO. I'd, again as I do with the DS9 jumpsuits, prefer that the ENT jumpsuits were a class C uniform and there was a class B jacket/pants combo uniform that is rarely seen. And Archer's dress uniform from 'TATV' wasn't bad.
They were probably just working off of modern US Military trends of streamlining uniforms. In the Army for example, they are working on cutting down the number of uniforms to two. Same with the Air Force, and the Navy is trying to slim down its un-necessarily massive amount of uniforms as well.