Given how business works, and I can't say if this is 100% true or not, but it seems to be the case from their previous decisions, but skins generally shouldn't be based on ideas the community uses. There've been numerous cases of fans taking those and trying to get compensation for the 'idea' that most big companies will avoid using anything close to what a fan has made without it being directly connected to something else in their library (like how Azir's necromancer skin FINALLY came out and was tied to Ryze Whitebeard, Caskgreaker Gragas, and Varus Swiftbolt). Additionally, Pool Party Illaoi would need to come out closer to when they release the Pool Party skins in the summer.
Unfortunately if you're getting annoyed at fan requests not getting into the game and them going with a different direction, it is partially due to legality. They want to avoid an incident similar to what happened to 'JuJu' in Skullgirls. For those that don't know its an incident where the team took the fan design for the character Juju who was only referenced in Parasoul's Super quoute where she said either "Take the shot Juju!" or "Number 13, take the shot!" This fan boasted about it and wanted compensation despite the NDA they setup for it and it caused a whole slew of issues before they just cut Juju's existence entirely.
There are MANY cases like this that have happened in the past. Complaining about them not using fan designs or even taking their own spin on things is because they CAN'T. Its a huge risk and it should be respectable that they may even attempt to release skins similar to fan ideas in the future. This isn't OG Riot who are a small group. They can't do stuff like French Maid Nidalee or Battle Bunny Riven anymore without a lot of risk and legal stuff.
Lastly, if Dunkmistress was one of the options do you really think anyone would vote for any of the other ideas? Let Riot flex its creative muscles. Most champs don't get a legendary skin as their second skin anyways, and we all know if Dunkmistress wasn't an 1820 people would get pissy about that :P