Github offers hosting of closed source projects, while Gitorious is all FOSS
Seems like a no-brainer to me. How many people develop absolutely nothing but open source projects that share 100% of their code from day zero? It stands to reason that a lot of programmers don't fall into that category.
(And even if I were paid to develop nothing but open-source software, I would occasionally want private repositories. I don't want to continuously display 100% of my prerelease code to the whole world. Looks like Gitorious has finally started to figure this out, years late: http://www.mail-archive.com/gitorious@googlegroups.com/msg01...)
And who wants to set up two Git hosts when one will do? (Even if Git does make it really easy to use more than one?)