Other than Japan, SK was the best bet among the G20 for getting a quick and easy trade deal win for the Trump admin. Trump's team has indicated they are willing to use defense agreements as negotiating leverage in these trade deals, and no where does that matter more than SK. The US and SK have a mutual defense treaty and the US has more than 15 major US bases (and a spattering of small ones) across South Korea. SK and Japan combined represent almost half of all US personnel stationed abroad, and the US spends about 4B/year on joint logistics in SK that supports SK troops. If anyone was going to be coerced to the table through defense it was SK. But with South Korea having a market with strict capital controls, and being a heavy export country, it seems almost impossible for them to make a deal that would meet Trump's demand list.