I once read an article in the Readers Digest on this kind of thing (so it must be true) but what stuck in my mind was one simple fact. The regional variations of you,yours, thee/thine and you/youm could be traced to a dividing line along the A5. I can't remember all the other nuggets but the logic became clear as it was explained.

Basically the A5 was an important road for nearly 2000 years and trade routes would have been along it far more than across it - particulalry as it pointed to London. And jurisdictions and policing would have had their demarcation along the route. So it would be no surprise that some aspects would polarize about such a landmark and in the case or you/youm thee/thine it was noticeable well into the 20th century.