Some accents are seen as uncouth (vulgar) while others are not, why?

As has been explained, the language used in Elizabethan court became the prestige because of its association with the social elite, so other dialects became defined by their contrast to Standard English. The dialects of the lower working classes became associated with the stereotypes of their professions, often seen as dirty, low skilled and uneducated. Correspondingly the language of these people came to be seen as inferior and in contrast to the more socially acceptable language of court. Furthermore, there evolved an expectation that those people with dialects perceived as inferior could not attain positions of importance, as shown in the Morrish extract with reference to Lord Denning and his west-country accent.