BHX vs. EDI
The growth pattern at EDI really started after devolution, when Edinburgh became a "proper" capital city of semi independent country.
Thereafter the growth in international business services moved away from GLA towards EDI, and aside of the LoCo operators EDI now has a good base of proper schedules, and also a growing inbound leisure market.
I don't doubt for a moment that if the UK was a federal country, with Birmingham as capital of say the Midlands you may well see better air links, such as you find in Dusseldorf (principal city in NRW), Hannover (capital of Niedersachsen) and Stuttgard (Baden Würtemburg).
Sadly that ain't the case, and with the eventual, and inevitable building of LHR's third runway, the business market is likely to become ever more Londoncentric.