[wperron]

There's a couple of reasons. Internally, there's quite a few things that we do in Rust that aren't actually taken care of by TypeScript -- Having our own LSP allows us to connect those bits (like the module graph resolver for example) directly to the internals of Deno, in-sync with the Deno version you have installed.

Deno also does more stuff than just providing Type definitions, embedding TypeScript and doing module resolution; it's a complete toolset -- there's things that we can cover having our own LSP that the TypeScript LSP can't. Linting, formatting, testing etc.