To use WSGI (which is what most people would use instead of mod_python) you have to write some lines in your server configuration plus a script that loads your framework of choice.

Compare that to getting CodeIgniter up and running: SFTP the files into the server, and you're up. No server configuration needed.

One could argue that this is not a feature of PHP itself, but that doesn't change the fact that "normal" PHP solutions are simply faster to get up and running.