Now that I think of it, this makes perfect sense as to why people don’t sing with their accents 

-One is technical. As Billy Bragg — a guy who's never had difficulty letting his accent shine through — explains, "You can’t sing something like 'Tracks of Your Tears' in a London accent. The cadences are all wrong." Different accents are often defined by their rhythms, intonation and vowel quality and length. For many accents, the tune and the rhythm of a song can constrain these qualities to the point where the accent seemingly disappears-. Click here for reason 2
(Source: mentalfloss.com)