Probably the biggest debate in all of the sport, people have been arguing whether to call the game soccer, or football, since the game was invented. Both are correct in whatever part of the world you're in, but it's true that only in the United States is the game exclusively called soccer.
When the game was first codified by the English, the game was named football. Because the English created Association Football, and wrote the rules referencing the game as football, the English call the game football. However, a popular colloquialism of the game was to shorten the term Association Football to just soccer, because the English language is never easy.
Why then does the United States use soccer? When Association Football made it to the US, rugby was undergoing experimentation that would later birth American Football. So, the term football was already in use. Naturally, using this colloquialism as the primary name for the sport made more sense than having two football sports.