Unfortunately, Popularity sells and it sells because they are popular. So, the more people continue to love champions like Miss Fortune, Ezreal, Ahri, Riven, ect the more we will see skins for them over champions like Kalista, Ornn, Skarner, Karthus, ect.
Kayn I'd say is a popular champion but the first skin he got post release was a Legendary, so that is a lot higher up than what most others get. Only Rumble had the honor of getting a Legendary as his first post release skin before Kayn, and he wasn't even nearly that popular. But when they do that for a champion it usually takes them awhile to get another skin out for them unless they are highly popular like say MF who got an Ultimate in 2018 alongside a variant SG Pajama skin, then a Halloween + Prestige Variant in 2019.