Erm, because you don't get a say? It will undermine democracy.
Hailed as a 'Devolution of Powers' from Westminster each devoluted Region or Combined Authority will have a Mayor, who would be ultimately held accountable and responsible. Why is one person to be a scapegoat for the decisions of many?
As an example, lets look at a South Yorkshire Combined Authortiy:
http://www.bbc.co.uk/news/uk-england-south-yorkshire-34414584
Updates and comments