Because sometimes, people want to have conversations with those of like mind without those conversations constantly being derailed, interrupted, and disrespected by outsiders. The debate areas are for that. The DIRs are not.
Of course, suspending (and, for repeated infringement, banning) those who show disrespect would have the result without the occasional aggravation.
Incidentally, I'm always amused by the "discussion, not debate" rules. As it says in the OED,
discuss Examination (of a point) by argument, etc; debate...
Or, for those USians who don't trust darned furriners, Webster's
New dictionary of synonyms says
Discuss, argue, debate mean to discourse about something in order to arrive a the truth or to convince others.
Discuss implies an attempt to sift or examine especially by presenting considerations pro and con...
Debate stresses formal or public argument...