Diana was the perfect opportunity to make a Blood Moon-based Legendary skin
Ashe and Yi had for Project line of skins. Veigar had for Arcade line of skins. Tristana for the Dragon Trainer line of skins. Sion for the Mecha line of skins. Cho Gath and Skarner got one for the Battlecast line of skins.
Hell you get it.
Considering that this champion should've had this skin a LONG ago by now, and that its based on the very entity she praises and that there are many Blood Moon skins around already. She was the PERFECT candidate and it was the perfect moment to make her a legendary skin based on this Skin line.
It puzzles me, I was expecting her skin to be a legendary :P