Skins are planned several months to a year in advance, and there is a certain amount of time that a champion has to wait before they can get another one, which is why we only get an Ezreal skin once or twice a year as apposed to with every release. All the champions chosen for PROJECT, as well as Arcade, had waited the appropriate amount of time since their last release to get another skin. It sucks, I know, but that's how shit works.