The MC did not forget about his mother. His mother was put on life support after she was inflicted by Eternal Sleep, and she has been in coma for a full 4 years, and since not a single soul has woken up after falling into Eternal Sleep, Jinwoo's mother is practically "dead" until someone somehow find the cure. As much as you love someone, as time goes by, you will be visiting their grave less and less frequently until it became an annual visit instead.
The reason Jinwoo is working his ass off as a Hunter, despite almost dying in every dungeon he entered, is to pay off his mother's medical bills, and also partly because he has no particular talent, so becoming a Hunter is the only choice to make enough money. Basically, Jinwoo has been doing everything he did for his mother, so no, he's not forgetting his mother for 50 chapters, but rather, he has been thinking about his mother in all his activities as a Hunter.
The main plot can only progress when Jinwoo is strong enough to bear that responsibility, because the main plot is way over his head for the Jinwoo from the past 70 chapters. He's getting closer to being strong enough now, and that's why, there are more and more hints of the main plot as Jinwoo becomes stronger. The plot will progress a lot faster once Jinwoo reaches that point.
As for why the author makes it look like the world revolves around him. I won't expand on it so as to not spoil you.