Theory: Why Luna Sells
I just noticed something: Luna only has TWO permanent shop skins.
Luna is a main support, a role that you need 2 of on every team even on a nonstandard team configuration. But not even all the strategosts are main supports. And she's one of the simpler strategists. So this main support scarcity ups Luna's pickrate. Its around 30%, like other main supports. New players are always likely to try her out because its extremely easy to notice you need a healer.
So they play her a lot and have VERY few skins to choose from in the shop EXCEPT for the limited time legendary that season. So we have a built in crowd of people buying Luna skins because they play her role, not because she's a favorite character from the Marvel catalogue.
People start to play Luna not for recognition but because they ONLY have a few options. Her pickrate is artificially inflated by the uneven role distribution that the devs are making worse with every dps release.
And then the FOMO philosophy on selling skins ARTIFICIALLY inflates Luna's sales rate because players can choose the shiny new skin, a recolor or two shop skins. These sales are used to justify a new skin next season over fan favorite characters that don't EVER get a decent skin release.
They are stuck in a logic circle.