Apparently it was a slang term originally. "Association Football" vs "Rugby Football" became "'ssoc-er" and "rugby". The term "soccer" became interchangeable with football in England around the time the US became more interested in the sport, and then the term became less popular and eventually not used at all in England.

America, however, established it as the "American Soccer Football Association", eventually dropping the Football term to avoid confusion with the already then established sport of American Football.

In conclusion, the British ruin everything they touch.