That does make sense. The README suggests that the goal was originally to also continue pulling new versions as they get uploaded to CPAN, and that seems to have not happened. ...and as the links others have provided explain, one big reason is how labor intensive the existing methods are; there lacks a fully automated process.
I can see how, even in its current condition, it can be helpful to an author who wants to pull old history into his own current repo. Perhaps the best thing that can be done to make Gitpan more useful in the short-term is to bring the README more up to date.