Cyrene won't be as popular as Firefly because Firefly is undoubtedly for (You) and can't be paired with any other man in the game, whereas people who like Cyrene always get cucked with Phainon.
Firefly enjoyers don't have to worry about shippings and cucking, because Firefly is indubitably yours, and 99% of her fanart is with (You). Meanwhile people who like Elysia and Cyrene have to step on a cuck minefield every time they search for fanart because of how much Phainon/Kevin cock she takes.