I've seen a lot of complaints about the same few champions getting skins all of the time. Champions like Lux, Ezreal, Ahri, Miss Fortune, Katarina, Annie etc. And there is some merit to those complaints, I mean I main Lux so I am fortunate, or unfortunate enough to be able to buy lots of skins for my favorite champion so I do speak, somewhat, from a place of privilege.
Most of the time, not all of the time, but most of the time Riot releases skins in a set these days, otherwise thought of as a theme. They have cosmic warriors, and ancient kingdoms, kpopstars, rock stars, sailor moon skins, etc. Why it's important to acknowledge that is, not every theme suits every champion.
Now, with that, there are a handful of champions that can slip in to nearly ANY theme and be successful. These are the basic humans, Like the ones listed above, Fiora, Cait, Ashe, etc. Champions who you can draw nearly any boy or girl, and paste it over them.
Some want riot to stop releasing skins for the easier to design for champions, and output more skins for those who are behind to catch them up to the other champions. There are a couple of issues with this;
Releasing skins for the sake of numbers is also a terrible idea because at the end of the day riot is a business and their wallet is what matters. Evening out mordekaiser skins over giving a skin to a person who is almost guaranteed to sell like ahri makes no sense.
Reasoning like this is why Ivern got dunkmaster... What even kind of skin is that, and Asols second skin is garbage too.
The reasoning is that these champions are much harder to find themes that fit them. Rushing a theme through because it sounds like a decent idea on paper is terrible because it results in an unfortunate skin and a net loss for the company.
Now I'm not saying that ivern should have had to wait as long as he did. But it's part of why these champs get so many skins all the time. They are so much easier to design for. Mf, Ahri, Lux, LeBlanc, etc. These are just women. They can fit into nearly any skinline that exists, with only their personalities to consider. Champions like Yorick, Ivern, Asol, Morde, Udyr etc. People with really iconic visual representation requirements either in their base skins or abilities themselves require a much more surgical approach to skins. You can't just slap Udyr into the star guardian line up, but nearly any skin line can fit on a basic human silhouette.
Lux, Ez, Kat, they sell skins. They make money. They are easy to make skins for.
What needs to happen, is they need to pick a champion and come up with a line AROUND them. Something that you can pepper the others into without it looking weird, but that fit those who don't have many skins now because skins themes haven't focused on them before. That takes far more thought than an anime skin for lux and ezreal though. Which is why we see them so often.