Faith Salie tackled this on WNYC's
Soundcheck a few weeks ago... there isn't a simple reason, but some theories range from the proliferation of American singers in global pop music and the American vowel sounds generally being rounder and more open thus more conducive to singing.
Why do accents disappear when we sing?
Words don't deserve that kind of malarkey. They're innocent, neutral, precise, standing for this, describing that, meaning the other, so if you look after them you can build bridges across incomprehension and chaos. But when they get their corners knocked off, they're no good anymore…I don't think writers are sacred, but words are. They deserve respect. If you get the right ones in the right order, you can nudge the world a little.