I did not know that EDI and BHX have roughly the same amount of pax p.a - but I would guess simply by the goegraphical location of the Scottish capital the majority of the services and therefore pax carried are actually to and from the London airports. BHX has a fairly wide choice of European destinations served and of course to further afield and the catchment area must be significantly higher than EDI....I still say Scotland would eb far better served with one major airport that could serve the needs of both EDI and GLA catchment areas -making best use combined services and possibly encouraging additional services.
Build an airport at Harthill Service Station........
Nivsy