Because not all scripting has strict performance requirements, and as much as I love JavaScript, Python blows it out of the water in scripting ergonomics and semantics IMO.

Opening / reading / writing files, parsing args, making HTTP requests, etc. which are extremely common scripting operations are easier with Python, and the end result tends to be much more succinct thanks to its syntax (this applies to all Python code compared to JavaScript, but I'd argue it's especially valuable in shorter scripts).