A saw an ad in an American radio station promoting Northern Ireland.
How do you see an ad on radio? Or do you mean you cut one in two in a radio station? This is not at all clear.
Why not one of the ME3 in Belfast? Well never say never but looking at some of the airports ahead of the combined NI throughput gives you the following and the dominant hub with ME3 connectivity.
STN / LTN / BRS (LHR)
LPL (MAN)
Oddly enough NCL does but EK don't serve EDI. DUB is just a much more commercially attractive given the fragmented market in NI. Also the balance between inbound business and outbound holidaymakers may also play a part.