Usually because the black hair isn't an abusive toxic man like Minho. The author likes drawing ukes in misery, past trauma and hard time finding love. Over Heat was is mostly about finding love because he's stuck on one guy, he didn't have trauma compared to the ukes in this story and melting point.
If he ends up with Minho he would never be happy and if with Taehwan it would always be them questioning their loyalty. It's best if they all separate.
Plus this is the author first love triangle and the author seems to like Taehwan more.
Dammit why are my replies sending before I finish what I'm saying. Anyway the author is currently making them all suffer in S3 so it's hard to tell what's gonna happen
guys this author constantly makes stuff where the pink haired character and black haired character end up together all of their stories r like that, so dont get ur hopes up