[jimmcslim]

Probably because he suspects that it will be more up to date than the PPA; however Michael DeHaan comments that Ansible themselves maintain the PPA so it will be as up to date as the PyPI version.

The other reason might be to support multiple versions of Ansible and its dependencies simultaneously, via virtualenvs.

[wldcordeiro]

That was what I suspected, but your second point is a very good one.