What is the point of legacy skins

There doesn't seem to be any actual logic behind the idea of retiring skins. For example, Dragon Knight Mordekaiser is basically just a recolor and it's considered a legacy skin, yet Infernal Mordekaiser is the same exact thing except it isn't a legacy skin. They were both released at the same time, on the same day, back in 2010.

And then you have Kennen.

Why does Riot make things legacy? Do they just not want my money?