Stockport and Oldham etc. are specific towns though, with a town centre.
Trafford is just a made up area with lines on a map that kind of trace the land the de Trafford family owned but it’s innacurate as a lot of that land extended into what is Manchester (Whalley Range and Moss Side, for example).
There’s no town or even a village called Trafford.
United should be called Stretford United because, while their ground is called Old Trafford, it’s not actually in Old Trafford (Old Trafford is further over on the other side of Chester Road), it’s in the Trafford Park industrial estate in Stretfrord.