Why does RP have a prestige associated with it?

The Standard English dialect has a long history of being used by the social elites, as demonstrated in the Mullen article, highlighting the percieved differences in dialects in the Elizabethan court. Although the difference between dialects was not necessarily seen as a bad thing, merely different, this difference became defined in context of the more "correct" Standard English used at court. This prestige was cemented in the 19th century with its standardisation in the public school system and has retained the title of "correct" English ever since.