There are a few reasons behind the switch, but the biggest one is that the ecosystem of tools and services around Git & Github is much, much larger than that around Launchpad & Bazaar. Already, by using Travis (thanks kampka!), I've been able to kill off an old, cruddy, insecure Jenkins installation on my server, and actually get proper testing across all of the Python versions that testtools supports. No more, "oops, I broke 3.2" again.
We still track our bugs on Launchpad, because Launchpad's bug tracker is still our favourite.