It also doesn't take much effort to make a thread, where as adding a new forum, I would imagine, takes more effort.
Given the serious nature of a debate forum, the moderation must also be maintained and kept rather strict, if not moreso than the other forums. That can also create more headaches for the Administrators/Moderators with either moderating it themselves, or finding others to do it. That also brings the question as to if it would bring in enough threads to warrant its creation, or if it would be used as seldom as the White Mushroom House.
Plenty to consider.