Because rust actually compiles the code to native binaries. C# kind of started doing it some time ago, but AOT still has various limitations and edge cases. Blazor doesn't even compile your code into wasm - it ships the whole dotnet runtime as wasm and runs your app as bytecode. This means massive downloads regardless how simple your code is.

There's relatively recent AOT for Blazor, but even that has to ship all the managed DLLs and part of runtime for the runtime reflection / metadata support.