I've been gone for a few years now and have no insight into this decision, so take anything I say with a grain of salt. Having said that, I think that, for better or worse, GitHub is probably the best location simply because it provides the lowest barrier to entry for new contributors.
I know that's spicy enough to trigger dozens of keyboard warriors hitting the reply button, but every little thing that deviates from "the norm" (for better or for worse, GitHub is that) causes a drop-off in people willing to contribute. There are still people out there, for example, who refuse to create an account on bugzilla.mozilla.org (not that this move to GitHub changes that).
I've been gone for a few years now and have no insight into this decision, so take anything I say with a grain of salt. Having said that, I think that, for better or worse, GitHub is probably the best location simply because it provides the lowest barrier to entry for new contributors.
I know that's spicy enough to trigger dozens of keyboard warriors hitting the reply button, but every little thing that deviates from "the norm" (for better or for worse, GitHub is that) causes a drop-off in people willing to contribute. There are still people out there, for example, who refuse to create an account on bugzilla.mozilla.org (not that this move to GitHub changes that).