If you bought a skin, and knew that it was only going to be sold for... say a month, never to be released again, and then because so many newer players wanted the skin so they released it again, how would you feel?
When you bought the skin, you thought that it was only going to be available for a month, and then suddenly it's available again.
Re-releasing skins would net Riot more money however Riot also values the "collector's sentiment" than many people can develop with their skins.
The fact that Riot hasn't made limited skins available is because they care about their community and know how much backlash they would get, making and skin limited would lose the meaning of 'limited'. Another thing that this proves, Riot listens to their community and tries to please as many people as possible (Bloodmoon Elise, Pink vs. Red) so since they haven't released skins like that, I think it just proves that the 'skins should always be avalible' mentality is in the minority.