Because English is the lingua Franca. ← Irony right there.
And none of the English speaking countries sound so musical as Americans. Our English is real. In singing our English is free of affectation. We pronounce the crucial letters. We speak from the heart, not through a screen of weird nationalized codified mispronunciations. To speak like an American simply drop all the bullshit and get real. And that's very good for music.
To speak like a British person pretend you're retarded. And that's not good for songs. So, British sing like Americans speak.
As individual singers they must be thinking, "Oh man, for my song to sound real I must drop all my cultural affectations, all my purposeful mispronunciations, all my class signifiers, all my purposeful culturally accepted mangling of proper English. Or else my song will sound as absurd as I do in ordinary speech. Get real. Get American.
When I first heard Adele singing I quite naturally assumed she was American country singer. Looked like one too. Then she spoke. Sheer bathos.
My own voice is destroyed.
Today I had to speak to a computer-driven menu over the telephone and could hardly croak out the words.
And yet the computer recognized my words. It didn't make me repeat anything.
Will I ever recover the dulcet tones of my natural voice?
I don't know.
It's as much my lungs as my voice box and larynx. All those things have to heal.