Iceland has a population of 330,000. It has developed, and is continuing to rapidly grow, large passenger numbers from a combination of destination and transit PAX, and from legacy (Icelandair) and LCC (Easyjet, WOW) airlines.
Scotland has a population of over 5 million, and EDI, like no other UK airport apart from LHR, is a destination. That is why its TA services are surviving when others from 'regional' airports are struggling. Perhaps if/when Norwegian arrive, and if EDI invest in pre-clearance, we could find that EDI becomes the KEF of the North? Why not?