Rioters i have a question

i have a question about how skins are chosen for champions. and do not get me wrong, im not saying like "make skins for champs with like only 3 skins" or whatnot. But i am wondering, what factors come into play when a champ is chosen for a skin? because whereas annie has over 10 skins or something and others have been around just as long yet have no more than 3 or 4 skins.

Thank you for any comments or information!