The key problem is that times change. A few years ago it was all about GLA and EDI was a wee local airport with all the TLC going west, when politics and the economy changed EDI overtook and left GLA behind to play catchup. The gulf between BFS and DUB is considerably wider than that. DUB is now in a different ballpark and the need for year round long haul to NI just isn't there.
Having Scottish route stop in NI would just have gifted UA's local traffic to Delta given they pay to fly non stop and not via Belfast. If one stop was acceptable, via LHR has a better hard product and lounges.