Because I know javascript a lot more than I know Lua (and I suspect given js popularity, a lot of people are in the same boat). Yes Lua is easy to learn, but it's still different enough that there is friction. The differences also aren't just syntactically, it's also libraries/APIs, and more. I also don't have any need/use for Lua beyond neovim, so it's basically having to learn a language specifically for one tool. It's not ideal for me.

But the people who did the work wanted Lua, and I have no problem with that. That's their privilege as the people doing the work. I'm still free to fork it and make ruby or js or whatever (Elixir would be awesome!) first-class.