why does the difference between American and British English exist? Because they have been seperated by geography and history, thus they have developed along different paths the same way Portuguese in Brazil is different from Portugues in Portugual.

Could you please clarify the question.

I would like to know the difference between them. I think I could not express myself correctly.

As Nick suggested, your question doesn't really make sense. "Why does something exist?" is a very philosophical question!