There was an old patreon post a while ago where KS basically said he made Yuka and Eri side characters so that he could focus more on a specific character's story. That's definitely a point of contention as Yuka is at least a commonly reoccurring (and seemingly well-liked) character.
People assumed Chika's story WAS that specific character's story. Another reason people call Chika a main girl is because of how she appears in that sort of flash forward at the beginning where it feels like it foreshadows the MC's harem. So it's a safe assumption to make that she IS a main girl. Plus she's a commonly occurring character and the main girls are usually shown quite often in these types of games.