because it's extremely easy to "deploy" on old-fashioned cgi-hosts. You need a python-cgi configured, drop 2 files with ftp and your app is up and running. Also auditing is a lot easier (and you only have to trust a handful of maintainers).
But if the question was why not split it up into smaller files (which it was), then that's nothing to do with the number of dependencies or the number maintainers. My point is that auditing a program with no external dependences doesn't get any easier if the code is contained in one file or across ten.
All the other reasons I heard, I don't really see what they have to do with having a single or multiple files.