There is no single, overarching reason but through years of observation and discussion of this with flairs, users, and fellow mods, I can certainly give a few common reasons, bth from my own experience and that of others':
The user(s) best able to answer is can't do so at that time, for any number of reasons. Everyone here volunteers their time to write because they enjoy it, but it isn't our job. Even the mods aren't hunched over our screens watching the new queue 24/7, and most users maybe check in once a day at most to see what questions there are. So in the first, we might just totally miss that the perfect question showed up. Or maybe we do see it, but dangit, we just don't have the time to answer it today! Or maybe we do have the time, but the source we absolutely need to do so was leant to a friend. Plenty more subreasons fit in here, but the short of it is, just because someone on here might be able to answer it doesn't mean they can at the time. Also similar in this is if there are multiple questions in the same broad topic. If, say, multiple questions about dueling were asked in a day, I probably only have time to answer one at best, so an overload of similar questions can also cause issue.
The question is not easily answered! Some questions ask about things where the answer is basically "We don't know", and the irony is that writing a "We don't know" answer can often be harder than writing a response where we do know, because it requires such a command of the literature to then be able to demonstrate the why of our ignorance. A big one that also fits here is the "why didn't X happen?" question, which we notices gets ignored a lot, because those are damn hard!
The question is uninteresting. I don't mean this in a mean-spirited way, but it is often the simplest questions which don't get an answer, we notice. If someone asked, say, "What was Gen. Lee's horse's name?", yes, I know the answer (Traveller), but I'm not going to be bothered to write an answer about the horse when it seems the users is just using us as Google by proxy. We don't relax the rules just because someone asks a question with a single word answer, but if that is the answer, why would someone bother wasting their time on providing the necessary depth to meet the rules? These sort of questions are, in fact, why we created the "SQSA" thread on Wednesdays, to try and help shift those to there, where you might be more inclined to get an answer.
The question is bad. Again, I don't mean this in a mean spirited way, we operate on a "No Stupid Questions" rule, but that doesn't mean questions can't be poorly phrased, unintelligibly phrased, questionably phrased, or rudely phrased... A question which comes off as demanding or entitled probably won't get an answer. A question where a user seems like they are just wanting someone to confirm their biases is likely not going to get answers, because we all know that if you do you'll likely just get argued with. Plenty of other reasons which fall into this category, but the real takeaway should be that it is worth putting some time into how you ask your question. We like answering questions for people who come off as interested and engaged.
The question is repetitious. We don't consider any question to be definitively answered, so don't ban reposts, but if you're the 10th person this month to ask is the Nazis were Socialists, you're probably getting ignored. There is an FAQ, and a search feature, and we really encourage people to use them. Sometimes mods or users will link to those to be helpful, but often it just doesn't feel worth the time (again, see above about being volunteers).
So anyways, that is a few reasons. Not an exhaustive list, but hopefully some insight.
Many reasons. I'm just a reader; a moderator or flaired expert can probably provide more details, but some of the reasons I'm aware of include:
The question itself is unanswerable due to a lack of primary or secondary sources or related information.
The question is so vague that it can't be answered with any degree of accuracy, or it's not a question about history to begin with.
The question is specific to a certain area / field of history, and AH doesn't have any or many contributors with expertise in that area. I've seen the moderators lament the dearth of flaired contributors in certain areas before, and I'm sure that often means that there just isn't anyone on-hand with the knowledge to write a comprehensive, well-sourced answer that meets the standards of AH.
The historians doing the (purely voluntary) answering don't have the time or the interest to research and compose an answer.
The question or variations upon it are posted so frequently (WWII, Hitler, Nazis, etc) that it's not worth the effort for someone to compose a new answer for people who don't search before posting. Often the mods or other users will link to similar posts that have comprehensive responses, though.
Nobody here is paid for their time. The moderators, flaired users, and other history experts posting answers are doing so voluntarily, and are not obligated to engage with every question that comes through the sub. Doing so takes time and effort, and if the question itself isn't particularly interesting, it's unreasonable to expect them to have to answer it just because someone posted it.
(Apologies if this is out of place; feel free to remove if necessary. Thanks again to the moderators and other contributors for all that you do to make AH such a great resource.)
There are a myriad of reasons behind this. The first reason is the complexity of the question being asked, and if there are any Historians / History Buffs with detailed knowledge on that subject currently present and willing to write a lengthy answer. Some questions we just don't have the answers to, for a lack of historical records or lack of study. Another reason is some question are pretty easy and or broad, and are covered on here frequently so might not warrant an in-depth response. Other times some questions are overlooked.
The vast majority of us here know a little bit about various times in history, but we are very skilled and honed in about a few topics. This forum requires in depth responses, with verifiable sources to back up your response. So us contributors are not going to comment on things we don't know, or have sparse knowledge. We try to stay in our lanes of focus. So like passing ships in the night, a question could be asked that an established commentor could readily answer, but he/she misses the question entirely because they weren't on reddit that day / week / month.
Edit: For example, I have traveled all over Africa for work, but only have historical knowledge of Africa in the imperial 1800s and 1900s sense; nothing before. So I am not going to profess and espouse claims about a subject I don't know much about - even if I've been to those places. But most things about World War II (specifically my flair) I and many others on here know a bit about, and can talk about it in length. So if you ask me about German South West Africa during World War I, I'd be able to answer, but if you ask me about ancient Namibia I can't. That's how this forum operates.
In addition to what the other have mentioned.....
The reddit ranking for top questions has a time penalty which is why so many of the questions at the top were posted in the past few hours. I don't see a question older than one day until the fourth page. Since it takes time to write an answer good enough to survive the mods, you're always going to see a bunch of unanswered questions on the first page.
The questions just keep coming. Endless questions, many of them the same.
In some cases the question is about something that probably didn't happen, but it's really hard to find citations to support a negative.
Hint: Google is good at indexing reddit and scrapes it several times a day. if you tack on the phrase site:reddit.com/r/askhistorians it will restrict the search to this subreddit.
e.g.
pink floyd site:reddit.com/r/askhistorians
Likewise you can specify -site and it will subtract results from a site.