While of course Miyahara will win, Yukihira is pretty much an angel. She's in love herself, but is stepping out of the way to let the other two be together even though Miyahara doesn't even know she wants that. Hard to imagine a better friend than that. So of course it's understandable that Yukihira has her fanbase.
Like these stories are usually about how the right match was there all along but the MC is distracted by some superficially popular character. But Yukihira is the real deal.
I think the biggest appeal for Yuki is that she DOES actually like MC that way, and frankly it feels unfair for her to have to step aside when Miyahara isn’t even trying because she doesn’t see him that way. Like I don’t hate her, but it leaves a bad taste in my mouth the idea that Miyahara might win without actually like, doing anything to win.
Like she’s fun, sure, but Yuki’s also fun. She’s thoughtful (kinda) but Yuki’s also thoughtful. Miyahara is also kind of annoying sometimes though, butts into things without thinking, and doesn’t really seem to put a lot of weight into MC’s thoughts and feelings more or less just dragging him along while Yuki seems to put a lot of consideration into MC.