Because people don't want to load 300MB for a simple website (and this is blocking the first render, not just loading in the background).

Not every language is a good source for targeting WASM, in the sense that you don't want to bring a whole standard library, custom runtime etc with you.

High-level languages may fare better if their GC is compatible with Wasm's GC model, though, as in that case the resulting binaries could be quite small. I believe Java-to-wasm binaries can be quite lean for that reason.

In c#'s case, it's probably mostly blazor's implementation, but it's not a good fit in this form for every kind of website (but very nice for e.g. an internal admin site and the like)