Because they are popular and sell really well. Plus most of them don't have multiple models to work with. Kayn has his base, his assassin and the Rhaast to design. That is 3 champions worth of work.
And as it has been said, his first post launch skin was Legendary which is like building a new champion, but in his case three new champions. It's also rare that a champion gets a legendary as their first skin, him and rumble being the only ones. So he will wait a bit till he gets another that they feel fits him or has been "requested" on the forums as many others have taken a long time to get skin concepts seen around here become a reality.