BuiltLikeTaft comments on Around what time did Americans stop speaking with a British accent?: However, if geography influenced some American accents to mirror some of the changes going on in British English, then it could also prevent them from occurring. Appalachia was much more isolated, and tended to retain its rhoticity. Its isolation also meant that it tended to keep a bunch of lexical features that fell out of standard English. One of my favorite is the past tense holp of the verb to help. If the major centers on the coast were keeping up with the Brits, places on the interior weren't.
It's also important to remember that much of the settlement of the US was done by non-English people. Pennsylvania was largely settled by German speakers, some of whom still retain a strong sense of German culture. Minnesota and the Dakotas were heavily settled by Scandinavians, Chicago by Polish and Irish (whose English was and is very rhotic).