it's bc the demon world is not a good place for nabi to grow up (can't go out of the castle bc humans are looked down on and preyed upon, and there used to be a law to kill humans on sight) + nabi's parents were looking for her the whole time since she was kidnapped by smugglers/traffickers.
skelly took some time to come to terms with that decision but he saw nabi's distraught parents and made up his mind to let her go and watch over her like this (as a cafe owner near her school)

I decided to look at the latest chapter, and I found out that he decided to give her up and all I’m wondering is why can someone who actually read the whole thing give me a brief explanation on why