Capcom thought that they made it clear when the game was released so I can think of two reasons why we don't know.
1: Alot of 'then why isn't's. If the answer is there maybe we missed it because people kept thinking 'that can't be because if that's so then why isn't this or isn't that' sort of train of thought. DMC4 is full of plot holes, people might not be willing to accept the answer because itself might be a plot hole.
2. Lost in translation: It's a translated script, maybe it was more obvious in the original Japanese.
1: Alot of 'then why isn't's. If the answer is there maybe we missed it because people kept thinking 'that can't be because if that's so then why isn't this or isn't that' sort of train of thought. DMC4 is full of plot holes, people might not be willing to accept the answer because itself might be a plot hole.
2. Lost in translation: It's a translated script, maybe it was more obvious in the original Japanese.