Because a large part the Python ecosystem uses packages (and stdlib modules) with C extensions and you can't easily transpile those. The end goal of Pyodide is

Because a large part the Python ecosystem uses packages (and stdlib modules) with C extensions and you can't easily transpile those. The end goal of Pyodide is to be able run existing projects mostly without modification in the browser, and that is not possible if one doesn't support Python C extensions. (I'm one of the maintainers)