Why does Maki and Nico always seem to be squeezed into every Love Live doujin somehow even when it's not about them.
To actually answer this question, I think it's a matter of story mechanics. When you want to explore a single character, one of the most effective way is by writing also about their "foil" — another character who shares some traits in a contrasting manner. (This is part of why rival or lancer characters work so well.) When the story is about a relationship, it's convenient to do the same thing, to compare to and contrast with another couple. NicoMaki is a relatively straightforward relationship, in their tropes of Tsun-to-Tsun combat and such, and they come from the fandom pretty much preformed and ready to pop in; you want to spend your time as a writer on the coupling you actually set out to explore, so not needing to spend effort establishing a relationship to compare against is a win.
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