Pardon for my "ignorance", but I'm curious about.......?

It was just the natural development of the language and dialect. But don't think there is a generic "American" accent or a generic "British" accent. Americans have Brooklyn, Chicago, Louisiana, and Southern accents, for example, while the British have their own accents, usually based on geographic regions. It's caused by variables such as origins of the inhabitants (French origins of the Louisiana accent and Italian origins for a Brooklyn accent for example) and isolation of the population (prior to media such as television and radio), which contributed to the Australian accent differentiating over time from what people generally identify as the generic London British accent..