Sure, I could fork Stack and implement features I consider important. But how do I get my Stack fork advertized on haskell-lang.org to reach as many people as possible?
You're implying everyone's equal. But the fact is that the haskell-lang.org team is made up almost exclusively high-profile Haskell developers who everybody knows already, including backing from a commercial entity which can promote it via their company site, as well as a book author who can promote haskell-lang.org via his new book. So while anybody could just do what the haskell-lang.org team did, it wouldn't have the same impact/effectiveness without a comparable endorsement.
So, some of the most high-profile Haskellers who have created vast amounts of Haskell software and documentation have started a new initiative and you, you're ... complaining?