The funny thing is if you look at who gets a lot of skins its lot less one-sided than it feels sometimes. For example, not counting Ashe, Sivir, or Kog cause of how long they've been out, you'd think the most popular ADC, Jinx, would have the most skins right? Nope. She has like 4. Jhin, a newer and also popular champ, has 3. Both are really popular. Additionally, none of the Battle Boss skins we got recently were for very popular champs. Brand and Malzahar are somewhere in the middle for popularity though Malzahar being very meta kinda boosts him past where he was when the skin was released. Only Ziggs is really that popular in the first place. There's Yasuo who still doesn't have many skins despite being hella popular and Lee Sin is an older champ and hadn't had a skin in a long ass time before he got God Fist.
Speaking of that, you mention Ryze, but Ryze isn't popular. He's more or less the original face of the game and is one of the originals. Of course he has a lot of skins, Riot was pushing him a lot back then. Plus he recently got a rework that updated all his super old skins. almost none of his skins looked that good prior to his rework so its not a fair statement to include him. I can't really defend Annie or Ahri though >_>
When was the skin Blitz got before these 2? Wasn't it Battle Boss? That one was awhile ago. I mean, if you had said Zed, Ahri, and Annie yeah you'd be right, but we recently got great skins for Shyvana, Gnar (another kinda sorta newer champ that's low on skins still), YORICK. It's a balancing act.
With how they do business now its best to release skins for less popular champs alongside skins for popular champs cause most people get the bundles, giving them a skin for a champ that may entice them to actually play and enjoy that new character.
Also basing your expectations on fanart is just asking to be disappointed. I posted another comment on the legality of it but another thing on that is that these are CONCEPTS. Riot only has a limited number of people working on skins at a time, so they can't be making new skins for everyone at the same time. Especially nowadays when they have so many yearly lines they keep up with its hard for them to focus down on singular skins that won't make anyone excited. That was the purpose of the vote. Besides, these options give them room to flex their creative muscles without people complaining that someone's fanart looked better than what they'd end up making >_>