In terms of EDI pax figures, domestic flights do account for a large chunk of their total which BHX will never have in the same numbers due to its much more central location and proximity to London
CAA stats show that in 2009 2.97 million passengers flew between EDI and the London airports (LHR, LGW, STN, LTN and LCY).
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).
And yet being the airport for the capital city of Wales hasn't meant a great leap forward for CWL.