Why do different parts of the U.S. have different accents?

Basically, it is because the populations become isolated and their way of pronunciation hardens into an accent. The basis for most Southers accents is the Virginia Piedmont accent. Over time, with other influences, it developed separately into the range of different Southern accents across the South.

“As human beings spread out into isolated communities, stresses and peculiarities develop. Over time these can develop into identifiable accents. In North America, the interaction of people from many ethnic backgrounds contributed to the formation of the different varieties of North American accents. It is difficult to measure or predict how long it takes an accent to formulate. Accents in the USA, Canada and Australia, for example, developed from the combinations of different accents and languages in various societies, and the effect of this on the various pronunciations of the British settlers, yet North American accents remain more distant, either as a result of time or of external or “foreign” linguistic interaction, such as the Italian accent.[3]”

“Children are able to take on accents relatively quickly. Children of immigrant families, for example, generally have a more native-like pronunciation than their parents, though both children and parents may have a noticeable non-native accent.[4] Accents seem to remain relatively malleable until a person’s early twenties, after which a person’s accent seems to become more entrenched.[5]

“All the same, accents are not fixed even in adulthood. An acoustic analysis by Jonathan Harrington of Queen Elizabeth II’s Royal Christmas Messages revealed that the speech patterns of even so conservative a figure as a monarch can continue to change over her lifetime.[6]”

Source

Most countries languages have many accents. One answer for you… location, location, location!

But then again… Why do I sound like my accent is from California when I’m from Hawaii??? When I was a kid I had a British accent, WTF? It faded away :(

People normally adapt to their surroundings of where they live. this also includes their accent. they call my state The Three States of Tennessee. west, middle and east Tennessee. the language accents are as diverse as daylight and dark from one location to the other. many television and radio annoucers come from middle Tennessee. these include Pat Sajack and Oprah Winfrey. this is because their accents are very suitable for entertainment.

It evolution and variation of the species by region.