Kayn waited over a year just to get a new skin, which was immediately a Legendary. Illaoi waited even longer regardless of being extremely relevant, same with Viktor. Fucking hell, this is ridiculous.
Also, as someone has pointed out before, some models are a lot easier to produce skins for than others. You've also got to take into account the work they have to do on particles for the champs where the same argument is also true.
Kayn takes 3 times as much effort to create skins for.
Riot markets around commonly played champs. Why do you think 
has so many skins?
Yorick waited six years, three failed reworks, one realized rework intentionally designed to bury the champ deeper, and an extra year before he got a really lazily conceived and designed skin that most of the hardcore fans were beyond disappointed with.
Urgot waited even longer and got an afterthought for his skin. (High Noon was all about Lucian and Thresh, while Urgot was just thrown in as an attempt to shut his fans up.)
Pyke will get his. It will suck, it won't sell well, and Riot will be able to focus on more Ahri and Soraka skins because TenCent demands more money.
