They weren't formed consciously, but regions - and even individual settlements - were much more cut off from one another in the middle ages than they are now. People would speak as they heard their family and neighbours speak, but Yorkshiremen wouldn't have much opportunity to hear how Kentishmen spoke (no radio or TV then to transmit speech patterns nationwide). Vocabulary and pronunciation do change gradually, even now, but they would have done so in different directions in different regions in those days.