Because WebAssembly is extremely limited? Unless you need to ship on the web, using WebAssembly is almost certainly a bad decision as it makes highly compromising tradeoffs that you don't need.
WebAssembly is a tool in work, supported by Google, Microsoft, Apple, etc... Currently it lacks a garbage collector, but this is planned. I would bet on wasm to be the holly graal in the future, more than anything else.
