I can only speculate, but based on the messages we receive from our followers, the usual reasons are the following:
You can’t prove that you can financially support yourself on this trip. You probably have insufficient funds or your sponsor is not in a good financial standing.
You can’t prove that you intend to return (rootedness). You probably don’t have a stable job at this point, newly employed, or newly resigned, and it’s giving the impression that you intend to work there.
Your intention/purpose in Korea is unclear. This is why it is best to be specific when you’re detailing the reasons for your visit.