Current standard JS runtimes (e.g. V8) are faster than current standard Python runtimes (such as CPython).
Not that Python can't be made faster (though many architectural decisions of the language resist it), but it hasn't been so far, so there's little incentive to include it to a browser. It gives too little new capabilities, on top of JS.
OTOH, say, WASM gave many new capabilities, and has been included.
JS was considerably slow until Google decided to spend resources to make it faster.