I have never quite understood why python (or ruby) packages are delivered through their own manager (pip, gem) instead of the local system package manager (apt, yum, etc). I mean I understand the python developer rationale and disagrees with it. My main problem with side-stepping the local package manager is that now the programming stack of prerequisites matters, a lot. It is not just about dependencies and APIs anymore but also about which package manager to use. That creates tons of problems especially when you rely on something with some very problematic design issues like easy_install.

If you are interested to learn more, here is a great post about the subject.