They sell skins which they know for sure will have buyers.
Lee sin, yasuo , vayne skins etc, are GUARANTEED to be hugely popular and since these are the most popular champions, and their mains (and a lot of other players) will buy these skins right on the spot.
For riot its not enough to sell 10 skins etc. They have sales targets that are probably in the six figures.
THats why these champions get skins, and will continue getting more and more skins. It makes good money.... much better money than less popular champions.
Its just common sense, if you sell a skin for a champion that MOST people play, you will sell more copies of that skin compared to a less popular champion.