As a Brit, may I ask this?

There are two types of "Football", Rugby Football (Commonly called Rugby) and Association Football (Commonly called "Football" in Europe and "Soccer" in Australia and the US) "Soccer" is actually an abbreviated form of "AsSOCiation", so calling the game soccer actually follows the same naming convention as used to refer to Rugby and is a less confusing way to refer to the game.

Because that is what the inventors of the game called it.