For a long time you couldn't actually fork it because qmail didn't have a license and djb refused to add one due his unconventional views on licenses and his general stubbornness. So the only thing you could do was distribute a "patch set". And all of this also meant it wasn't packages in many repos.

By the time he finally added a copyright notice it was kind of a "too little, too late" kind of affair.

This is the story with most "djb-ware": daemontools, djbdns, qmail. I think it's a real shame because all of these had great potential to be picked up by others after djb himself lost interest. I suppose daemontools is the most "successful", but only in the form of the runit re-implementation.