I think Yi, Tryn, and Ezreal are unfair examples. They have been around so fucking long that skins are going to be made for them back when there werent very many champs to make skins for in the first place,
Ezreal has been around since 2010. Skins were released in 2010,2013,2014,2015,2016,2017 (and the 2013 one was TPA) (and the majority of his skins were released in 2010. (4+base skin)
Tryn was back in 2009. so same reason as Ezreal. Example of this for him. Skins were released for him in 2010,2011,2012,2014,2015,2016,2017 (he got like 1 skin every year. He is also got his name from the owner of Riot games. Riot Tryndamere which probably means he gets a skin thrown his way every year for that reason alone.)
Yi was in the first week of alpha. skins were released in 2009,2010,2011,2012,2015,2017
Yasuo, Jihn, and Ahri are definately popularity ones, but the others. They have just been around so long that its kind of reasonable that they have a lot of skins