I mean to be fair, it's one thing to just say a name like "Dreadnova Urgot" but it's an entirely different thing to actually make that work and look good while fitting the thematic and overall visual guide of the game. Even if they drew 500 awesome designs for the skin they'd have to pick one to put in the game, and if they can't decide on which one they want it could fall through completely.
And that's just for one theme, they also have to decide which theme to actually go with. Ultimately they're only making 1 skin for that champion at that moment so they have to decide which theme is the strongest. Just in this thread alone you can see different people gravitate towards different concepts, they'd probably be a bit upset if Riot didn't pick their favourite theme. You see a lot of comments like that when they reveal new skins for champions that don't get skins all that often, "looks like I have to wait a couple of years for a skin I like" or "I guess this will have to do for the next couple of years".
All those awesome fan concepts are visions of what the artist themselves wants out of a skin, but Riot has to design a skin for their entire player base (or at least for those who play that champion). That artist might love yellow, but will a yellow skin actually sell well in the game? Even if you look at the most requested skins (like Dragon Master Swain) there's tons of different designs from different artists because they all have different visions on how that skin should look. When that skin eventually comes out a lot of people will inevitably be disappointed because it didn't look how they wanted it to look.
It's harder than it sounds to design something for the masses.
