I think you've captured the reason exactly. When you have to pay your own way, you're more inclined to look for value for your money, and Korean has reasonable fares combined with a very good up-to-date economy product, especially on many if not most of its flights to/from the US. I know that KE's well-priced and (reasonably) comfortable economy service between IAD and ICN came as a very pleasant and welcome surprise to me.