What's preventing it is their existing codebases, mostly.

IIRC, one of the first things Neovim did was throw out literally tens of thousands of lines of legacy code from Vim.

Meanwhile, Helix can just add the `lsp-types` crate as a dependency and they're already a quarter of the way to making LSP work.

The difference between adding something to a 30-year-old C project and a new Rust project is so massive that it can actually be the difference between "fairly straightforward" and "essentially impossible".