Riot won't do Blood Moon Diana because it's redundant
I can't possibly think of other redundant skins they've made.
It's not like:
is from Ionia,
is already a cowboy,
is already Draven ,
is already Mundo,
is a pirate,
is a robot, a robot, or a robot,
is a bird made of dark ice, or
is blue with dark purple clothes.
And we won't even get started on re-textures or recolors.
But hey, at least no one else has a Lunar Goddess-like skin.

Edit: Maybe that's not the reason they refuse to do it, but it's been confirmed that her only skin in the works has been the new Infernal one.
They've shown a tendency to throw away great ideas in the past. They could've easily made and marketed a Blood Moon skin for her during last year's actual blood moon, but they missed the chance. People on the champion skin team don't come to the boards much or accept fan-made ideas (even though this one is based off an existing skin line). Just look at how long it took them to make Pool Party Zac, despite the fanart for the skin having existed since his release.