Because in chapter 82.5 (where there is the authors notes), they say that Eun-ho and Rak-Hyun has all the personality she has preferences, so it's just to imagine that they would make a better story for the second couple than for the main one.
In the author's words (ch 82.5): "Eun-ho and Rak-Hyun were some of the most talked about and troublesome characters I've ever created...! Two characters packed with all the elements I personally love."
So for me (after reading this) it was just to be expected that these two would be more well-written than Hansol and Woobin, because when you have two characters that you like working on more, you tend to make them more interesting, especially if they have many toxic traits.
Rak-Hyun is a punk and a brat, while Eun-ho is a slut and a piece of trash (author's words not mine). And based in their whole story, they do fit better as a secondary, just because of the conflicts with Hansol (who is the reason they started talking).
We all (or most) can agree that the second couple is definitely better than the main one. Hansol is indeed stupid and annoying, and the way the author is writting the story now, makes it look that Eun-ho and Rak-Hyun is more the main couple than Hansol and Woobin, but shouldn't we all expect that after the story had ended?