Shed, very true those aren't capital cities (though many are regional capitals within their countries) however with the notable exception of Frankfurt and Zurich the places you list all have a population in their metropolitan areas more than twice that of Manchester - in the case of New York (x 3?) nearly four times, another significant factor which airline route planners take in to account. They are also generally much further away from their nearest city 'competitors' and so have an even wider catchment area. By the time you throw in the fact that roughly 20% of the population live in the London metropolitan area the chances of securing a more extensive long haul network - or as you rightly point out a based long haul carrier - reduce even further. The advent of the B787 might help in the coming years but other regional airports may be hoping the same thing.
Changing subject, can anyone shed any light as to why the fire cover at MAN is only NOTAM'd as CAT 10 (i.e. sufficient to cover A380 ops) until late October? Surely it should be a permanent change - assuming the A380 operation isn't just seasonal?