Yorick in particular is in a really bad spot, his concept was really good at the time but in practice it lacks counterplay and is really strong at all points in the game if you know what you're doing. Rito has supposedly been working on a rework on him for years, and he's been removed from free week rotation for a while now. I think Rito just wants to keep as few people playing him as possible since strait up removing a champion would make a lot of the fan base really angry.
Other champions like Zac, Azir, Gnar, or Udyr (I know he just got a skin but he didnt get one for AGES before that) all have a need for multiple skins or at least partial extras (EX: Standard Udyrs cape or ball Zac), Azir needs both his soldiers and his turrets for any skin to look half decent on him.
Rito does also show lots of favoritism over who gets new skins, just look at Annie (10 skins) or Teemo (9 skins) compared to Urgot (4 skins (I never even knew Butcher Urgot, ButchUrgot?, was a thing)) for proof of that.