Please take any and perhaps even all of this with a grain of salt. But, if what Sukishoo alluded too earlier is at least partially accurate, and I believe it is, then the new theme for Blood Moon appeals more to an age old tale that every champion could have.
"Fear the wilds for you may be ensnared by the Spider Queen!"
"There is a great price for vengeance...and a greater price for betrayal."
"The Warden still haunts those very Isles."
"The Eye of Twilight watches us all to ensure balance among all else..."
I agree that Kennen, all said and done, should have a Blood Moon skin, but when I think about Zed from a lore perspective it doesn't make as much sense as it does from a theme perspective. It would be cool to have a Blood Moon Zed skin, but can we break things enough to make that work? Wouldn't it be too obvious to have a Blood Moon Zed skin (Lunar Eclipse Zed? ...maybe...no? I have no idea.)? Should we really want Shen (Akali, and Kennen) all buddy buddy pal pal like he didn't kill Shen's father?
I'm not saying it doesn't make sense to have a Blood Moon Kennen or a Blood Moon Zed. Those would be cool. I'm just saying that from a "What story do these champions have to tell?" perspective we're (surprisingly) better off with Blood Moon Zilean for now.
The story of an immortal man with a past as shrouded in mystery as the battle that "created" him. How many lives has he touched in the shady bars of Piltover? His role in establishing Bandle City? Just what did he advise Azir to do all those years ago? How many has he saved from Rek'Sai's ancestors? And perhaps a very plausible question to mull over, who was the wise man that came to Ionia to suggest an order based on preserving balance...such that their civilization does not fall to ruin?
Zilean could be many things...there's really no telling. You can do a lot as an immortal...or at least I would imagine, haha.
Edit 4/8/2015: As for the argument for all the "Champions that should have gotten a Blood Moon Skin", maybe something better is in the works. Also, you have to think about the "Age Old Tale" perspective. Just what warped and twisted stories do they have to tell? I will admit that many of them have a very interesting lore to back that up, but still....right now I can't see how that would beat an immortal man shaping the very world the other guys live on.