And isn’t Wasm supposed to (some day) free us from the need to pick JS for the front end?
In practice though the main reason is that to have decent DOM bindings you need to stabilize many other specs first (unless you do a ultra-specific DOM-only extension, but nobody wants that)
Rust, for example, has a huge package with interfaces for every DOM function.
But I wouldn't recommend it in general, because JS (and for slightly more complex things, TS) is much easier to create because of all the old Rust features that surface every time you mention it. And most GUI code is simple enough that you only have to write once, maybe fix a few details, and forget about it.
Wasm would be much more compelling if it was target by higher level languages.