Another factor with the Blood War is that they're incapable of trusting each other. It's not necessarily as complicated as xenophobia or as simple as Law vs. Chaos. It's standard villian in-fighting. If the Demons decided to conquer the good-aligned planes, they would leave themselves open to attack from the Devils, and vice-versa. There's no way they can trust each other enough to turn their backs even for an instant. So one has to defeat the other before they can turn their attention to the rest of the universe. Part of that is law vs. chaos, because they can't fully understand each other, but the larger part of it is just the nature of evil. When two of the defining characteristics of your existence are deceit and betrayal, it doesn't engender a lot of trust.
