Let me flip that question. Why not Glasgow? Glasgow as a city is a very popular tourist destination in its own right. The behavior of Korean tourists in Scotland will be similar to when Brits visit Asia in that very few will be visiting just one place. GLA is perfectly placed as the gateway to Loch Lomond (which is very popular with Asian, Middle Eastern and North Americans) as well as the highlands and islands. You can be on the "highland" railway within under an hour after landing at GLA. GLA is without question the best placed airport in Scotland for visitors coming to see Scotland's scenery.
Nobody is going to sit on an aircraft for 14 hours to fly to Scotland just to visit one city. So this business was as much GLA's to win as anyone else's.
And with the service appearing to be such a success clearly Korean Air and the associated tour operator made the right decision.