With the close links between LM and KAI, the upcoming USAF T-X competition (featuring the KAI/LM T-50) I am not surprised that Korea is leaning towards F-35. It doesn't sound exactly like a free and fair competition but when are these things ever fair?
Regional politics come into play too, a case of keeping up with the Jones (or Japan in this case).
Anything but F-35 would appear to be the wrong answer....