When it was set up, many years ago, it was actually for "am I being unreasonable?" questions and was set up because asking AIBU became a popular thing to do. I suppose it was like the "ask me anything" topic in that respect - it was set up as a natural home for a new "trend" of threads. MNHQ moved everything that wasn't an AIBU out of the topic and into Chat where it generally belonged.
I just think it's evolved past that now.