Not using it increases the barrier to entry. Potential contributors are likely to know git and be relatively familiar with the contribution process on Github.

Hosting it elsewhere means people are less likely to find it in the first place.

Using a different VCS means people are less likely to pick it up. Instead of starting on their contribution they have to install and learn new tooling that is irrelevant to the project itself.

Emacs is a pretty good example of this. There are plenty of people who would be interested in making improvements to the project but a much smaller proportion are willing to engage with the email-based process that's required to do so.

The crux of the issue is that the social element of an approach is just as important as the technical merit. A technically superior solution will quickly languish if no one can bear to work on it.