Because releasing it into the public domain would benefit people more than it hurts Disney? That's not the right question to ask anyway. Copyright only exists

Because releasing it into the public domain would benefit people more than it hurts Disney? That's not the right question to ask anyway. Copyright only exists insomuch as government grants it - thus they can take it away if they please, or after an arbitrary duration. You should really ask how long is reasonable for a creator's work to be protected, and just how many generations of his descendants should inherit that right.