The theory (a very sound one, based on internal observation) is that the strict "answers, not discussion" format is what attracts and retains high quality participants.
I really strongly believe that it's an either/or proposition, you either get discussion or you get quality answers to questions. Discussion "sucks the air out", so to speak.
And since there are already so many places for discussion on the internet, it's better for us to focus on Q&A.
No knock on SE. Quite the opposite.
But I frequently have questions that aren't SE style questions, and I wish I knew where to take them for such intelligent and informed responses.