I've thought about the British-American singing voice a lot over the years, and I think I have a part-explanation: the slight drawl comes from relaxing the voice and allowing the words to flow more than in speech. Most of the US accent comes from the English West Country accents spoken by the early settlers, but the drawl is also a significant part of it, making speech (and song) sound more American ten English (unless, I presume, you come from the West Country).

Steve