I watched several documentaries over the years that said it is because at the time Football Clubs were referred to as Social Clubs and the team was just part of the Social Club. The clubs were referred to as "Socs" pronounced like the footwear Socks, and the Teams would play what was referred to a s "Soccer" as in Sock-er. Then this got exported to various people in North America, mostly from the South and West of England. Then it fell out of usage in England but no one told us over here in North America so we kept using the term.