Why did this tank?
It seemed like it had a lot going for it -- a great cast, pretty much everyone in a significant speaking role is a well-known (if not also well-respected) actor, the plot is this combination between Shakespeare and a Greek epic with a really interesting mythology of its own.
You can critique Vin Diesel, but he is perfect for the role he has and the rest of the cast can't really be faulted as actors.
I can only guess that it was too much of too many things. Too much intrigue for your action fan, not enough science in its science fiction for science fiction fans, and maybe not sophisticated enough to pass as palace intrigue drama.
I often wonder what kind of success this might have had if the film hadn't been tied to the Pitch Black film. It might have made the first half more coherent and allowed for a bit more introduction to who the Riddick character was. I also wonder if it would have worked better as a 2 or 3 film series, with the first one being mostly about Riddick and the bounty hunters, and the second one tackling more of the political intrigue.