Asian movies in general stand out (to people that don't live in Asia) because the motivations for all of the characters are very different. It can be very refreshing to not see the same retarded tropes that you see every day, which are generally really hard to relate to (for me). Why Korea in specific, I have no idea. Although it is important to note that there are also plenty of good Chinese movies.

Korean industry is far more horizontally integrated than western industries. The Korean entertainment production industry is far more tied with their consumer product verticals.

They don't need a given series to be a smash investment if they control the actors/actresses/clothing choices/product placements, etc.