Gonna give my two cents, since, well, look at my flair.
It's known that players who main these low-pickrate champions are what you could call "true" enthusiasts; it doesn't matter if the champion is garbage or not, they keep playing because they genuinely like the champion.
In fact, that's why, at least prior to his mini-rework, Aurelion Sol had one of the highest winrates in midlane, despite his pickrate being one of the lowest. The only people who played him were people who played him frequently, because they genuinely liked the champion. Aurelion was stuck with his release skin for quite a while, until Riot relented and gave him Mecha, which is incredibly underwhelming; I don't know for certain the "buyrate" for the skin, but if it's low, it's completely understandable. But I can assure you, if he got a good skin, it would sell.
Compare with Kai'sa, for example, who has one the highest pickrates and winrates of all adcs ever since her rel...