You guys need to realize something: Riot makes skins to make money. Skins are most likely their main source of income since the game is free. Riot also has all the data they need to decide who to make skins for. They'll take into account how many people play certain champions (and how often they are played), as well as data such as average sales per skin for each champion. If a champion with 5 skins sells 20 times more than a champion with 2 skins, it simply makes more sense from a business perspective to budget development time/money to the first champion.
Sure, they might not be making skins that people ask for. However, the amount of people asking for skins for a specific champion is probably insignificant compared to the amount of sales per skin for another champion. It would be like McDonalds deciding to sell almond milk simply because a single family of 5 asked for it. It just doesn't make sense from a business prospective.
Now, I'm not saying I know everything about how Riot operates, or how they choose skins. This is purely logic and surface level business analysis. I would love to see what others think, or even some insight from a Riot employee.