This is an interesting and thoughtful meta question, and it's worth thinking about.

Broadly speaking 'why did X not happen' questions is a pretty broad category, and in many cases, those questions are possible to answer - sometimes they're just 'why did X happen?' written differently, and can be approached as such.

We're aware, of course, that some questions of this nature are hard and perhaps impossible to answer. Interestingly, looking through the list of questions I've answered, none of them actually answer a 'why did X not happen?' question as far as I can tell.

We mods can deploy a macro in such threads that goes like this:

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!

Additionally, as /u/sunagainstgold alludes to in their response, 'why didn't I learn about X in school?' questions are common, and difficult to answer unless you're an expert in educational history (and even then, I don't doubt that /u/uraccountabilibuddy groans at those questions too). And we have a macro for those too:

Hi there! You’ve asked a question along the lines of ‘why didn’t I learn about X’. We’re happy to let this question stand, but there are a variety of reasons why you may find it hard to get a good answer to this question on /r/AskHistorians.

Firstly, school curricula and how they are taught vary strongly between different countries and even even different states. Additionally, how they are taught is often influenced by teachers having to compromise on how much time they can spend on any given topic. More information on your location and level of education might be helpful to answer this question.

Secondly, we have noticed that these questions are often phrased to be about people's individual experience but what they are really about is why a certain event is more prominent in popular narratives of history than others.

Instead of asking "Why haven't I learned about event ...", considering asking "What importance do scholars assign to event ... in the context of such and such history?" - the latter question is often closer to what to what people actually want to know and is more likely to get a good answer from an expert. If you intend to ask the 'What importance do scholars assign to event X' question instead, let us know and we'll remove this question.

Thank you!

Which is to say that, yes - these questions can be annoying and difficult to answer, and we have these macros because we wish to prepare OP for the possibility of disappointment. But answers might happen nonetheless!

My very strong preference is to ban "Why doesn't X country/region teach Y," that's for sure.

As for "why didn't Z happen," the mods in the past have generally rolled our eyes but figured it allows for a dedicated person to explain that History Does Not Work That Way--there isn't some pre-existing mold that things have to fit into ("why didn't Q region develop cities") and flip the question around.