Riot - Why can I not buy the chromas at any time if I own the skin?

I've found it incredibly frustrating and just plain stupid that if I own a skin like Headmistress Fiora, Winter Wonder Fiora, or Bewitching Janna, the chromas are not in the shop. Why? They're dependent on owning the skin to begin with, so if you own the skin, it makes no sense as to why the chromas are still locked out.

Why are people ok with additive content for skins being time limited? The skins themselves are already limited, so why should the add-ons to the skins be separately limited, and not tied to whether or not the skin is available to your account?