Because you lot lost the war. 
Depends on which war, and who you are talking to. If you are referring to the British, and WW2, technically the British and their allies won the war against the Axis powers. Admittedly, without help from the Americans, it would have been difficult.
Still, English as of today is a fragmented language. Americans cannot (or will not) understand British English. And British English has a lot of difficult dialects. Not to mention Scottish, Singlish, Australian, and other variants. Non-native English speakers in particular have a hard time relating to the different variants.
What is really needed, is an English variant of
Académie française, that could act as a global advisor for usage and spelling of the English language. It should probably be based on BBC English or
Received Pronunciation.