Because deb and rpm repos lag behind releases, install packages globally, and don't lend themselves to portability. Setting up a private deb repository is still prohibitively complex even for those of us who only target one platform and architecture (at least last I checked).

Why we as a Python community need Bento is another question. In my mind distribute, pip, and virtualenv combine to create a simple, robust, portable packaging, building, and installation system that I have been very happy with. Dependencies are automatically resolved, environments are self-contained, and portability/backup is a copy command away.

I love aptitude for installation of global dependencies; beyond that I've never found a use for it.