Personally, I think it was because Yami was insecure and believed that Yuu and Hikari would be better off together, regardless of her own feelings. I think it’s likely that Yami was the one who broke up with Yuu, and not the other way around, since it’s said that Yami was trying to avoid him after their breakup and he was looking for her. The discourse between Yami, Hikari, and the twist as a whole has sorta made people forget that Yami and Hikari are still best friends (before the kiss atleast) and Yami still cares about Hikari as her friend. So, in her mind, she probably thinks that Yuu deserves to be happy with someone “better“ than her, like Hikari, and Hikari deserves to be happy with Yuu, especially when both of them seem to have feelings for each other. So she breaks up with him, so that Hikari and Yuu can be together, not necessarily because she and Yuu fell out of love with each other. This is just speculation, of course, so it could really go whichever way. I do wanna say, though, that I don’t think any character here deserves a “bad” ending. I’ve seen a lot of hate towards the three character, mostly Yami and Yuu, and I feel that it’s pretty unwarranted. At the end of the day, they’re all just sorta inexperienced teenagers trying to figure out their emotions and feelings when it comes to romance. I don’t even think it’s necessarily bad writing either, the author clearly cares about this story if he’s willing to release a web novel AND manga completely for free.