I think it's the second. It's certainly not the first. Sorry for generalizing, but for whatever reason, amateur historians (usually guys) get really into the details of military history.

Look at the shear number of pop military history magazines (some admittedly defunct) in this list: https://en.wikipedia.org/wiki/Category:Military_magazines. While some of these are undoubtedly not historically oriented and focus on the lifestyle of enlisted men/women and the like.

Military history is growing slowly as an academic discipline, but more often than not, scholars focus more on the social aspects of warfare. This isn't to say that there are purely military historians out there and that their work is invalid, they're just not terribly common.

Definitely the second. Military history is only a small sub-field of the academic discipline of history. For historical/historiographical reasons, historians of other areas often consider it old-fashioned, trivial and boring. As /u/Enrico_Dandolo points out, modern military historians tend to look at social, economic, religious or cultural aspects of warfare. This is partly because there is much more to be said on these formerly neglected aspects of military history, and partly because their area of interest needs to be qualified and justified in an academic environment that is usually disinterested in "straight" military history (weapons, armies, tactics, strategy etc).

Among the general public, on the other hand, military history seems to be considered one of the more spectacular and fascinating areas of history. This generates a lot of publications from amateur historians, as well as providing professional historians with a ready audience for their more accessible, popularising accounts.

I can't claim to know why interest in military history is so widespread, but I could speculate on a couple of points. One is the way it connects to a range of other hobbies and interests - reenactment, wargaming, tabletop games, modelling, video games, movies etc. Any of these hobbies can draw people into military history as a subject of study. Another is the fact that the traditional historical narratives of many nations (including the ones we were all made to learn in school) tend to focus very much on the great military leaders and events of that nation's past. Most people are never introduced to all that much history, but they will definitely have heard of things like Julius Caesar or WW2. Another reason is the common association of war with the sort of violent spectacle that we (especially young men) seem to enjoy as entertainment. Economic history may affect more people in more profound ways, but it's sorely lacking in explosions and death and thrilling heroics.

I agree with the others here that history buffs, especially men, tend to focus a lot on military history.

I don't think that is a tendency unique to this sub or even to reddit. The stack exchange history forum shows a pronounced interest in World War 2, and warfare more generally.

Or look at this subforum about asian history and note how the threads with the most views and replies are directly related to warfare. That is a particularly striking (cherry picked) example, differences aren't always that striking. Of course, that forum has an entire separate subforum dedicated to military history as well.

And of course, there are forums specifically focused on talking only about military history.

Podcaster Dan Carlin has gained quite a following creating a product that focuses a lot on topics like World War 1, the Mongol conquests, the political instability and civil war at the end of the Roman Republic.

As a final example, when the History channel focused on history, a large part of their programming focused on Roman military and engineering, barbarian invasions, Nazi secret weapons, and the US civil war, though there was other fare such as history of drug prohibition and the presidents.

I think it is fair to say that military history looms very large in popular history. Partly, that is due to the way history is taught in high school. In the United States at least, American History lessons focus a good deal of attention on the Revolutionary war, the Civil War, and World War 2. Lessons about these wars tend to have a good deal of narrative about what political ideals caused the sides to fight.

Similarly, an introductory course about ancient Greek history could be expected to discuss the Persian wars, specific battles like Marathon, Thermopylae or Salamis, events that caused the wars and ideals that motivated the Greeks to fight (at least according to Herodotus).

So, there is a factor of people having some foundation of knowledge about certain wars, which both increases their likelihood of having a specific question and expecting an answer to be available. And, having lots of questions to answer means that knowledgeable experts and aficionados have lots to work with and stick around.

Conversely, if someone is wholly ignorant of a subject, they are unlikely to ask questions about it. People don't tend to get many history lessons about African pre-colonial states, and so they don't tend to ask questions about African pre-colonial states. So, people who are knowledgeable about African pre-colonial states don't tend to get questions to engage them, and they don't stick around. When a question does arrive, there may not be anyone around to answer it, unfortunately.

I think a quick browse of most general, non-specialist bookstores tell the same story. The "MILITARY" section is usually as big or bigger than the "(non-military) HISTORY" section. So, it's the second.