Because Prestige has nothing to do with who deserves a skin or how old the champion is. It literally only has to do with money.
Popular champions that sell skins are who get prestige skins. They invest the extra time and effort into making a shiny version of the money-maker skin in an event because that's what's cost effective.
Why do you think the vast majority of prestige skins are for female champions? Because boobs sell skins.
Fizz got one because he was the star of the dog skins and he summons a giant cute dog. Thresh is going to get one because he's one of the most popular supports with a lot of mains. Same goes for Lee Sin.
They aren't gonna make Prestige Maokai or Prestige Bard because they don't sell skins. Their mains are few and far between.
Money is the only motivator here.