*pull out AP human geography text book*
The forms of these different dialects are formed usually by relocation diffusion to new regions, by isolation these area form a branch of their language. This is determined by region, but popular culture does have an effect in the whole mess. And with the advancements in communication, it's forming a somewhat bond within different areas.
P.S. That map is somewhat incorrect, it is forgetting "soft drinks" and it's labeling is a bit weird with the percentages.