How did the way we pronounce words become so different?

I work in a very diverse workplace, we have many countries represented within our workforce. Individual’s country of origin is easily recognisable by their accent, “When people listen to someone speak with a different accent from their own, they notice the difference, and they may even make certain biased social judgments about the speaker.” This is because someone’s socio-economic status can often be identified by their accent. eg. The stereo typical “plum in the mouth” enunciation speech trait of upper class English.

This got me thinking, how does everyone in one country pronounce their words the same? If you have a good ear you can not only work out what country someone is from but also which city! As a child learning language you obviously hear the accent of those around you and pick it up. I don’t understand though how the accents became so different, for example Australians have a very dry, flat accent compared to the Southern drawl of someone that speaks English in the USA.

Following is a link to an archive which “presents a large set of speech samples from a variety of language backgrounds. Native and non-native speakers of English read the same paragraph and are carefully transcribed. The archive is used by people who wish to compare and analyse the accents of different English speakers”: http://accent.gmu.edu/
The following link is an archive of accents and dialects of the UK:
http://www.bl.uk/learning/langlit/sounds/index.html

Anybody know?