Right, but the HotS skins aren't cannon either. I think you misunderstood when the OP said lore skins, as they meant the skin itself had lore. The whole point of the blurb is to explain the flavor, or rather what situations/world could exist in which these skins were cannon. There was a little bit of this with the Void Fizz skin release:
A chill grips the man’s spine as a warped giggle echoes through the streets. He’s coming. Fizz used to be welcomed with open arms but recently, something’s changed. Vaulting over walls with his corrupted trident, he’s been terrorizing everyone for weeks with his demented games. The man starts running, passing by an alley when something strikes his back and drags him to the ground. Reality tears beneath him and a sickly sweet odor envelops the street. He looks back and sees Fizz, reeking of corruption, staring and chuckling. Moments later the ground erupts as a putrid leviathan bursts forth from below, devouring the man in an instant. Void Fizz and his pet love to play hide-and-seek.
The idea isn't that Fizz has in-cannon become void-ified, but rather this skin represents a reality where he might have (and the whole "multiverse"/"skin-cannon" thing is already an idea that's been heavily suggested with the Battlecast universe).
Obviously it wouldn't need to be this long, as the HotS example is only a few sentences. The difficult part comes in actually creating these snip-its for the hundreds of skins you have, as well as where you'd actually be able t put the blurbs. On the champion page itself? We already have enough issues with the client holding text that I don't think that's really feasible; and does anyone really go to the site's champion pages? Is it worth that effort?
So, yeah, it's a cool idea. I would love for it to happen, and I'm sure Riot would like it too. Adding flavor to the Lol "multiverse" (HA!) is something I think every player can get behind. The question is: is that time and effort spent doing something that only adds flavor to an admittedly non-cannon aspect of the world for every instance? I'd say no, but perhaps there's a compromise in there by only focusing on particular universes and fleshing them out. This in turn provides a lot of inspiration for the art/skins team. I know the Battlecast universe is one of those that a lot of people have suggested interest in seeing more of, and even some of the other more neglected skin themes could see some life and love pumped into them.
Just some thoughts.