I don’t have some grand theory on SO’s downfall. All I will say is that sometimes us programmers try to optimize every single thing in life and that’s simply not how humans work.
SO tried to optimize the Q/A by eagerly rejecting repeat questions, without considering what the consequences of that were. If you’re in a classroom / lecture hall, your teacher / professor isn’t going to chide you for asking an “obvious” or “repeat” question (bad ones might). Imagine if every time you wanted to talk to a TA or professor you first needed to lookup previously asked questions in a database and then, iff your question hasn’t been asked before, you may ask the professor or TA after hours. Programmers might call this system “efficient” but most people would be pretty unhappy with it.