I didn't really see a clear explanation. Even Nokia had a public bug tracker for Harmattan! (Well, before it died out together with the system and Nokia's Linux efforts in general). Yeah, that can be tedious to manage, but it's the only functional way to engage community in the bug reporting, which also allows to see some progress on the bugs. Otherwise there will be a lot of unnecessary duplication, and inefficiency, which is even worse than keeping 2 bug trackers.