Ultimate Skins

With Riot being such a large company with a game that has a huge playerbase, why are there not more high end skins available? DJ Sona is amazing, and I personally love that kind of thing, and would drop money on an ultimate skin if it was available for any of the champions I play. Even legendary skins are fairly scarce compared to the number of champions. Seems like the cost of development on these would be quickly offset by the sudden sales spike.