Apparently the Birmingham referendum is going to get a drubbing and so are Salford and Manchester. The government are talking about making city mayors into regional mayors, so that there would be a mayor for Greater Manchester etc... rather than just a small section of the city.
London gets the attention because London is the economic growth engine of the UK. When London powers ahead so does the rest of the country, when London fails, so does the rest of the country, and while the position of Mayor is largely symbolic having someone like Ken in charge would send out a poor message that London is closed for business, all IMO of course.