For better or for worse, it is the dialect of 19th century economic and social elites that became the prescribed standard for all. This is not surprising, as only these elites had the resources to create and colleges and universities, and to educate their children in them. Moreover, they owned the publishing companies responsible for textbooks, grammars and dictionaries that promoted the spread of a standardized dialect through out the nation. They therefore label all other dialects as ungrammatical. No dialect is any more or any less grammatical than another.
Dialects are merely different in their grammatical rules, but they all have rules.
For example, in Letcher County one such rule is that verbs ending in s are plural, while singular verbs do not. So has is plural, as in Brad and Caroline has a baby girl. This happens to be the opposite of the rule in the standard dialect of English found in textbooks, but it is no less grammatical or rule governed. It is simply a different dialect. It is necessary for young people in Letcher County to learn to speak, and especially to write, in the standard dialect.