JavaScript being garbage-collected, having a rather complex runtime, not having proper integers etc... makes it a rather unfortunate compile target. Yes you can do it and it is done but js idiosyncrasies leak into the source langues pretty easily.
Well the user perspective is: suddenly there a compilers for all kind of languages targetting the browser because wasm makes it easier for the maintainers to build them.
Sure you could have implemented unsigned integer semantics on top of bool-arrays in javascript years ago but if the result is too slow you have gained because the compiled c++ code does not run fast enough to be usable.
