Why have all the previous colonies of the British (Empire) developed their own regional/country-specific variation of the English accent, as opposed to following the uniform ('Queen's English) English accent?
For example, the Australian English accent differs to the one in Canada, as is Hong Kong and India.
Accents change over time (listen to a news broadcast from the 1950s and compare it to a present-day broadcast to get some kind of idea of how accents change), and all the colonies' inhabitants were isolated from each other to some degree, so their accents changed in different ways.
I'm sure there is a very similar thread like this in the FAQ.