He was very sick and needed surgery that can only be done in the states. His sickness was getting worse to the point he was forgetting the name of kwon's mom. So he left without telling her because he didn't want to burden her of taking care of him. He didn't know at the time that she was pregnant.
That's already been explained in the last few chapters, for more precise details please read about last 10 chapters. But if you don't want to here's a brief summary: he had brain tumor or something and he didn't want worry kwon's mom, so he left to get surgery without telling her, be assumed it won't take long to recover and he'll come back quickly. But complications occurred and he lost most of his memories so it took him this long to finally remember his life in korea and come back. But kwon and his mother didn't know, so they assumed he just abandoned them. Hope this helps, this is what I remember I maybe wrong about a few details.
After he got surgery he entered a coma followed by amnesia and took him decades to regain his memories and also how to walk and stuff
Can anyone plz tell me why kwons dad left them in the first place ?