One might not give a second thought about this but accents melt into general neutrality of accents when singing. This evaporating of dialects and merging into one neutral accent goes beyond the British Isles and other non-American musicians. The most common one being the Swedish band ABBA, Danish band MLTR, German band Scorpion or even for that matter Americans with heavy hilbilly accents singing in accents that they don’t normally use when conversing. You are at a Coldplay or Adele connect and after the show comes a buff, very Cockney accent. So, what is the science behind this disappearing of accents when singing?
While there can be various reasons that accents ‘disappear’ in song, the most obvious reason has to do with phonetics, the pace at which they sing and speak, and the air pressure from one’s vocal cords. The reason behind this automatic flip to ‘American’ accent and not some other accent, it’s simply because the generic ‘American’ accent is fairly neutral.
According to linguist and author, David Crystal, a song’s melody cancels out the intonations of speech, followed by the beat of the music cancelling out the rhythm of speech. Once this happens, singers are forced to stress syllables as they are accented in the music, which forces singers to elongate their vowels. To put it in simpler terms, it’s the pace of the music that affects the pace of the singer’s delivery. A person’s accent is easily detectable when they are speaking at normal speed. When singing, the pace is often slower. Words are drawn out and more powerfully pronounced and the accent becomes more neutral.
However, this is not to say that all singers with their all accents sing in ‘American’. We can take example of the British band, 1975, Arctic Monkeys, Sex Pistols, etc which you can clearly tell they're British. The same goes for American country singers themselves such as Johnny Cash, Dolly Parton who even sings in her East Tennessee accent.
Last but not the least, it is worth keeping in mind that the American English is not the most natural, non-accented form; that’s why the language is called “English” not “American”. Meaning singers don’t shift to American accent, it's a specific accent range that suits the style of music.