It's an interesting question, that can perhaps be answered somewhat accurately by reference to stereotypes about your average Redditor. However, I think an interesting dynamic at play here is about the nature of asking questions, particularly the specific requirements we have of questions here on AskHistorians. Put simply, asking a question tends to require a baseline of knowledge. You can't ask about something you know nothing about - you need to have a good enough frame of reference to be able to know what you want know more about. If you told me to ask a question about Chinese history, for instance, I'd have precious little to work from - aside from 20th century stuff, I'd perhaps have the Boxer Rebellion, Opium Wars, Hong Kong and Mongols to work from, a set of topics that leaves out vast swathes of history and, you'll note, tends to involve the intersection of Chinese history with other histories.

This means that the most common questions are always going to be on subjects for which there is a significant degree of common knowledge across many contexts. The Nazis fit that mould - they are common reference points in education, popular culture and politics across many contexts. Particularly in Europe and North America, they lend themselves to incorporation into national syllabuses - the Second World War left a mark on a great many societies, offering a way to connect local perspectives to bigger historical events. Simply put, the vast majority of Redditors have enough background knowledge to formulate a vaguely sensible question about Nazis, in a way that isn't true for a lot of topics.

One perk of spending enough time on AskHistorians though is that you build the knowledge you need to ask better questions over time. One of my favourite things that shows this is looking at the questions that our 'Interesting Inquirer' flairs have asked over time - the quality of the questions they ask tends to just get better and better as they go on, to the point that they end up asking absolutely fantastic questions on a regular basis. My suspicion is that just about anyone who asked, say, one question a week and read the answers would have a similar trajectory - but we'll always still get plenty of questions about Nazis from people encountering the sub (and the in-depth study of history) for the first time.

If think the emphasis on "here" is a bit misleading. WWII and by extension the nazis are a very popular subject in (popular) history period. It's not just Reddit. If you browse your local bookstore's history section or just check out Goodreads or a TV channel that has documentaries, WWII will be popular.

As to why that is, I can think of a few reasons.

First of all, WWII and the nazis certainly fall into the "Extremes of the human condition"-Side of history. Murder violence and extremism are just more interesting to a mainstream audience than the intricacies of the Ancienne Regime's tax code.

Then you have the Hollywood-Aspect: there's just lots of movies TV shows and other entertainment media that (for better or for worse) deal with WWII. There's just no Steven Spielberg Movie about Ghengis Khan, so your average jane has a vastly smaller chance of being exposed to that area of history through pop culture.

Then I think there's the recency aspect. The nazis and WWII are one of the earliest big events in history where movie and audio footage from the time exists. I think that is a huge factor in how attractive some area of history can be to the mainstream. It just helps a lot if you can show footage from the time, and not just have to rely on recreations.

Lastly, I think there's the personal-aspect. For a lot of people alive today, they can point to some family connection to WWII and the nazis. Particularly with Americans, a lot of them have some ancestor that fought in WWII or escaped from Europe. If you have a personal connection to something, even if it's remote, it will probably be more interesting to you.

One thing I want to tag onto the excellent analyses here is that gender is also in play with all of this. WWII is a very, very popular historical topic ... among men. Reddit is often a rather gender-segregated space, and while there are a number of subs that are primarily populated with women (makeup/fashion-related subs, subs for certain fandoms, etc), the history subs are frequently majority-male spaces, as shown in the topics that turn up repeatedly and get highly upvoted.

The equivalent of stereotypically female historical interests would probably be either the Tudors, the English Regency, and/or Victorian fashion - all of which also get disproportionate attention here in comparison to other royals/periods, it's just that because of the skewed gender ratio on the sub they do not get to quite the same level of disproportion as WWII.

I remember watching an interview with a couple of big-name British historians on the BBC right before the 70th anniversary of the start of World War II. I want to say it was Richard Overy and/or Richard Evans but don't hold me to that. What I do remember is that they said they were frankly surprised that World War II has had the staying power in popular imagination that it does.

I think there's a few reasons for this, to build on the comments given here. A lot of it is frankly political. As official political ideology has largely fallen by the wayside, Russia and China in particular have leaned pretty heavily into their World War II history with a lot of movies, television series and documentaries made since 2000 or so that focus on the era. But it's not just those countries - British politicians still reference the "Blitz spirit" (even though as Overy has pointed out, that is largely a myth), and in the US there was a huge upswing in interest in World War II in the 1990s and the 50th anniversary, but also with Tom Brokaw's 1998 book The Greatest Generation, which came out the same year that Saving Private Ryan did. To be honest a lot of that US interest was connected to generational politics, as referenced by Brokaw's title: many Baby Boomers were coming to terms with their parent's generation and their role in World War II (which kind of overlooks older generations' participation actually) as those people were aging and beginning to pass away, and often re-evaluating their lives despite the sometimes more cantankerous intergenerational relationships of previous decades.

There are other reasons for its persistence in media internationally. It's a "good" war with (apparently) clear moral sides. It was a conventional "total" war with set-piece battles between the world powers in a way that really hasn't been seen since. It's also both within living memory still, and relatable in a way, but also safely in the past and over - we don't really live in fear of an upcoming World War III in the way people did even some 40 years ago, nor do we quite live with the immediate results of the war anymore, with occupying powers' troops staring across a border in Germany at each other while parts of Dresden were left in ruins. So I think there's ironically a bit of escapism in World War II's popularity: it's easier to play a video game set in World War II or watch a zombie movie set during D-Day than it would be to do the same with similar media set in contemporary Somalia, Iraq or Afghanistan - people do try this from time to time, but it's much more controversial.

A final point on World War II media is connected with what was already mentioned about a baseline of knowledge. World War II media has something of a positive feedback loop going with popular knowledge - you can utilize it for movies, television shows, video games, you name it and people will already understand the context (or a particular narrative about the context). The Captain America film pretty much did this without really having to explain hardly anything about World War II at all. For both American domestic and for international audiences, I don't really think this is true with hardly any other historic event, both before World War II or since.

ETA - a postscript. I think a lot of this also has added urgency in people's minds because frankly neofascism is a serious political force in a way it wasn't even 20 years ago. I remember having a political science seminar with one of my most hated professors (he laundered money for Iran-Contra, so he was kind of a jerk) circa 2002, and him throwing out that fascism as a serious political ideology died in 1945. I don't know if he thinks differently today, but plenty of other people clearly do, whether they consider themselves adherents, opponents, or observers.