Because nerds watch BBC, and pick up on lingo from British tv shows, such as "fap" and "ginger". It isn't an American term, I literally never heard anyone use it until within the 6-7 years or so.
"Redhead" was always the term anyone usually used. Now it's hip to use "ginger", because I guess to some people, it sounds better, and takes a half-second less time to say. Me personally, I've always found the term slightly annoying.







