Ohh I wonder why he abandoned her
This is from google: "What was South Korea like in 1970? Living in South Korea in the 1970s could be described as very poverty-racked and unsophisticated, or, in other words, a lot of Korean people lived a simple yet rough life."
Either the guy (and his wife) wanted his daughter to have a better life, or he probably lost his wife and was too overwhelmed to raise a child as a single father, so he gave her up.
These are just my guesses though
Been following this every update