If you want more high-end skins for less popular champions, then there needs to be more demand.
Developing skins costs money.
If it's not going to give back a meaningful profit, there's no need to bother investing in a 1350/1820 skin for a champion.
YES, I FULLY SUPPORT THOSE WHO PLAY SAID LESS POPULAR CHAMPIONS WANTING SKINS, BUT THIS IS JUST PLAIN ECONOMIC SENSE.

crying in the corner
doesn't get a skin because she's not uguu kawaii waifu, nyaa. Just because she's a woman whose frame isn't based off of Lux's, she got kind of a meh opening skin and hasn't even gotten touched since.