Because up until relatively recently, JS was slow and crippled. The public image of JS is that its only use is Ajax and making popups. It's only recently being recognized as a "real language". Surprising and note-worthy that you can do ML in JS? No. But novel? Yes.

That you can implement them isn't impressive.

But most ML algorithms ("applying a pretrained neural network" being a notable exception to that) are compute-intensive, so in a language that doesn't have access to Lapack/Blas/etc, performance will be terrible.