Because every country has their own language, their own games, their own traditions.
If an American ask the same question to another country person may be the question is, " Why Soccer is called Football everywhere else than America."

Football Game: http://www.koffii.com/GameDescription.aspx

Soccer Game: http://www.koffii.com/GameDescription.aspx

In America and Australia it's called Soccer. Here, we have our own football, it's called the Australian Football League, and it seems really pussy to Americans because their football is all eggheads ramming each other and it looks tough. Soccer seems pussy to us because you could snap the players like a twig and they look like pussys chasing the ball. We have a good-ly balanced sport.

So, three different kinds of 'football'. Ours is the only real one, and the only good one :)

Not only in America.We call it soccer in South Africa, but Fifa also accepts it as such.That's why they have a Soccerex (for soccer expo)

Because it is cumbersome to say 'American football' every time, so we picked up a British slang term to describe that silly sport we don't play where every other game is a tie.

American football is known as "gridiron" outside of america.

Some people refer to gridiron as a "steroid sport" although this include baseball aswell.

How about this, what do you call american football in england?