Why DO some British singers sing....? It's because the American pronunciation is the natural way to speak. Most American people don't have accents. Some people - like Brits - have what is known as 'accents'. People who have accents have their own different way of saying things. But when they sing, they relax, so the words come out normally. That's why Brits lose their accents when they sing. But then, when they start talking, their accent comes back. Weird, isn't it? ;)

I agree with Coligno

My theory is that, as in ancient Greece certain dialects became associated with certain literary genres and thus writers would tend to write in the dialect appropriate to the kind of text they were composing rather than using their native dialect for everything, so in English certain musical genres were first invented and/or popularised and/or dominated by American artists, and when British artists later got in on the act they tried to give their music a more authentic sound by imitating the articulatory precedent set by their American precursors. It has nothing to do with the American accent being easier to pronounce or more relaxed, as I have heard some people explain it in the past. @Su.ki. I suppose you're joking.

Because they want to make more money

because they like it