Who's going to moderate it?
I
was nominated, but don't want to find myself on the DOXing end of a BS lawsuit. And it's not a subject I would enjoy spending a lot of time on.
To protect against BS lawsuits, it would be good if the Mod has always used Tor.
Less people would join though, mainly because
1. You can't see the discussion without register/join. Some people would like to see what's going on before decide to register or join.
For the same reason, a .onion forum won't get the exposure it deserves.