[derefr]

Presumably because discussion forums aren't anyone at Github's expertise. They're a whole product of their own.

[hasenj]

This argument could be made against wikis and issues and static websites (and file uploads).

I think a discussion forum is integral to any open source project. People traditionally use mailing lists, which have an awful UX in my opinion. A moderately better alternative is google groups, which is basically just a nicer UI for a mailing list.