I have also noticed this. I have spoken to a friend of mine who is a professional singer and he said it comes down a lot to training. If someone has had singing lessons they are often taught certain ways of singing that comes into a more middle ground which often sounds more mainstream and American, likely due to the popularity of american artists in modern popular music.
You will often find front men in bands do not do this like the previously mentioned Alex Turner from the Arctic Monkeys. They have often just formed a band and started singing and gone along with it.
The same thing often happens when you sing in a choir. The accent is removed.
Just my thinking on it. Very interesting topic.