Because it's quick and easy to radically alter and refactor your prototype as you learn the problem space. By the time it works you often find out that you don't need anything more. This is something that Perl had.

Once your program starts to get bigger you have abstractions that can cope fairly well and keep your code simple to use - this is what Perl didn't have.

If you need more speed then you can write extensions in some compiled language.I think TCL was better at this hybrid approach but Python is a nicer language in itself.

You can also just dump python and write everything in that other language but now you understand the problem space quite well and you won't be trying to learn about it using a language where change is "difficult."