I think it's because the Yanks got hold of it, so calling Football, soccer sounds like an americanisation of the name. The truth is that the nickname soccer comes from the old name of Association FootballWhen did soccer/football become such a shibboleth in this country? I'm sure when I was a small child in the 90s it wasn't seen as that weird to call it soccer, and I can't imagine things like Soccer Saturday would be called that if it had been considered such a big deal, notwithstanding the obvious appeal of the alliteration.