So Kaisa has like the double amount of skins that Kalista has, but Kalista has existed for many years, while Kaisa only existed for one year.
Yeah, Kaisa gets more skins because she is more popular. Champs that are more popular get more skins.
Thing is Riot doesn't give a shit about some champs. Xerath, Azir, Kalista, they're all champs Riot literally has forgotten about, and that is why they're not popular.
Of Riot would actually care about more than 20 champs, more of them could be popular