I dont know why he has changed his singing style since Oasis.
His vocals used to be be fantastic - unlike Ian Brown for eg whos a great front man but simply can't sing.
Nowadays.... Well I think we can all hear that his vocals are pretty horrible.
He seemed to have deliberately changed to shouting nasally words.
Even his stance. If you look at his early live performances he sang with his chin up which opened up his throat more. Now he's almost singing down into the mic.