Performance. JS can be as fast as wasm, but generally isn't on huge, complex applications. Wasm was designed for things like Unity games, Adobe Photoshop, and Figma - that is why they all use it. Benchmarks on such applications usually show a 2x speedup for wasm, and much faster startup (by avoiding JS tiering).

Also, the ability to recompile existing code to wasm is often important. Unity or Photoshop could, in theory, write a new codebase for the Web, but recompiling their existing applications is much more appealing, and it also reuses all their existing performance work there.