In any modern web application you most probably will end up using Javascript for the frontend part, and you have plenty of choice for the backend, which actually includes some really good options both in Javascript and Typescript.

So choosing Python makes you swtich languages between FE and BE.

Then if we are talking about the BE, Javascript is usually more performant, there is bigger choice of libraries and frameworks, and the tooling is simply better ( and improving).

And then there is the async story. Yes Python has a way to write async code, but is more like a patch on top of fully synchronous language rather than full solution, not to mention the very crippled syntax of Python that doesn't let you write anonymous functions.