The answer to this can be found in Terry Gilliam's introduction to The Imaginarium of Doctor Parnassus on DVD and Blu-Ray. The main reason behind this is the death of Heath Ledger. Here are Gilliam's words: "The loss of Heath Ledger was such a blow that it was a kind of a test of everybody involved in this film. Whether, in these kinds of incredibly difficult circumstances, we could continue and make a film worthy of Heath's last movie. And I think because of so many people, so many people who loved him, respected him, we pulled it off. It's also why I couldn't put a credit that was contractual on the film. And the contractual credit was, 'A film by Terrry Gilliam.' And I couldn't do that. I just didn't think that was right. And so one night, the cast and some of the crew were all gathered in Vancouver, toasting Heath, and saying, 'What are we gonna do? What are we gonna call it?' And we all agreed to call it, 'A film from Heath Ledger & Friends.' And I think it's truly the honest, accurate, and right credit. Forget contracts, for once let the truth speak."