Just a suggestion here that we don't get too hung up on the 'capital/regional' label. Many of the world's most successful airports are not capital city operations, eg. Chicago, Frankfurt, New York x 3, Sydney, Toronto, Atlanta, Milan, Zurich, Munich, Barcelona and many more.
What really counts is the business opportunity offered by the catchment area feeding a particular proposed route. The population density, the GDP of the region, ease of airport access, disproportionate ethnic demand for certain routes (eg. MAN-ISB). These are typical elements which airlines examine, not capital city status (although some capitals do offer an enhanced wow/glamour factor). Does anybody here expect Canberra, Bern, Ottawa, Cardiff (or even Washington DC) become global superhubs anytime soon? "Capital City" refers to the seat of political legislature and is just one factor in the attractiveness of an airport.
Many capital cities are superhubs, of course. London being a case in point. Perhaps it is more important to be the home base of a major airline, as is the case with LHR (BA) capital city, and FRA (LH) provincial city. The "capital" factor is not the overriding one, although taking the case of London in particular as a 'global city', there are many iconic tourist haunts, big business HQ's etc. related to capital status which boost its attractiveness. But the region surrounding MAN is also relatively prosperous by global standards and has much to commend it to airline companies.
But MAN's Achilles Heel is that it is not home-base to any major airline company. None of the major alliance groupings hub in any significant way at MAN. These are the true advantages which LHR, FRA, MUC, CDG etc hold over MAN. Note that two of those airports serve political seats of government and two do not. The "capital city/regional city" label is a sideshow distracting from the factors which actually drive airport success. MAN has many positive factors to commend it to the airlines. Unfortunately, being a major alliance hub is not one of them. That is why we will not match MUC's offerings in the foreseeable future. LH supports MUC as BA dismisses MAN.
SHED.