This could be a wide ranging topic.
Why do English singers typically lose their English Accent when singing? I listened to a discussion about the brain on National Public Radio and it asserted that our brains work differently when talking as compared to when singing. I found several links on line, here is reason 2 sounds good to me:
www.daytranslations.com
In casual speaking, everyone has full control over how to utter words, as unwittingly guided or nurtured by environmental factors. These environmental factors are mainly the usual people a person converses with and the usual conversations a person is exposed to. Verbalizing when speaking is different from verbalizing when singing. In the former, it’s the thought or the message being conveyed that counts. In the latter, it’s the melody and beat that matter. Talking is the same regardless of the intonation, speed, and rhythm by which a person speaks. With singing, a song is not going to be the same song without the correct tune and beat.

Why Do UK Singers Seem to Lose Their Accent When Singing?
It’s one of those questions you probably wanted to ask but you keep forgetting. Why don’t British singers sound British when they sing? Adele, who has a distinctive London accent, sounds American in her hit songs, notably different from the way she normally speaks. Irish

Reason #2: Because of the melodies and beats
Another interesting, and perhaps the more convincing theory, is that British singers lose their accents because of the melodies and beats they are trying to follow ase they sing. Linguist and author David Crystal said that the melody of a song neutralizes intonations while the beat takes away the usual rhythm associated with speech.In casual speaking, everyone has full control over how to utter words, as unwittingly guided or nurtured by environmental factors. These environmental factors are mainly the usual people a person converses with and the usual conversations a person is exposed to. Verbalizing when speaking is different from verbalizing when singing. In the former, it’s the thought or the message being conveyed that counts. In the latter, it’s the melody and beat that matter. Talking is the same regardless of the intonation, speed, and rhythm by which a person speaks. With singing, a song is not going to be the same song without the correct tune and beat.