There are many things github issues is not capable of:
- Integration with Bamboo
- Review workflow
- Fix verification workflow
- Detailed search
Github issues might seem convenient for developers in the beginning, but I doubt it'll help users of phpBB in the long run. Based on our experience with phpBB3, features like a sophisticated search as well as statistics are rather useful. We are using Jira for phpBB3.x so this would mean that we would have two separate trackers for developers to check. Most of the phpBB community is already used to Jira now, so switching yet again to another tracker seems like an additional deterrant to contribution.