People tend to write dev tools in the languages they develop in. C compilers written in C, JavaScript minifiers written in JavaScript, etc. There's often no reason to use the same language for the tooling, but you use what you know and like to work with, and if you've got a need for a language-specific tool (like Webpack) you're probably going to start out writing it in the same language.

If Rust becomes popular for JavaScript tooling, how long until those developers abandon JavaScript entirely? This isn't necessarily a problem, but it could lead to some, if you can't fix your own tools because you're not conversant in the language. More importantly, if the key toolmakers all switch to Rust and lose interest in JavaScript, the tool authors might lose touch with (or interest in) the problems of the JavaScript community. We all talk about dogfooding, and this is the opposite.

Rust seems to have this cult-like power. Rust developers are definitely drinking a lot of Kool-Aid. Maybe if all the JavaScript tools are written in Rust, we'll start seeing Rustscript in the browser.