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 that are common knowledge across contexts. Nazis and Romans both fit that mould - they are common reference points in education, popular culture and (especially for Nazis) politics across many contexts. Particularly in Europe and North America, they lend themselves to incorporation into national syllabuses - the Roman Empire, and 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 or Romans, 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 Nazis and Romans from people encountering the sub (and the in-depth study of history) for the first time.
I think the one thing you can add to the positive feedback loop of "if I ask this, it will be answered" that doesn't require disparaging the outlook or perspective of Redditors per se is that Nazism and classical Rome function as top of the line analogies, metaphors and similes in much of contemporary life. As a result, many people are not just familiar with them but are curious about whether the common uses of both histories are appropriate, accurate or misleading both in how they comment on contemporary life and in how they create an understanding of both periods.
A common point on this that I often find myself making as someone not specifically expert in the history of Nazism but as someone with a strong interest in modern states comparatively is that the common use of the Nazi state as a metaphor for ruthless efficiency is partly a consequence of scholarship and intellectual commentary on the bureaucratization of the Holocaust, particularly the Wannsee Conference and partly a consequence of Nazism's own propaganda, but that this is exactly wrong--the Nazi state, especially before the beginning of World War II (and in other ways afterwards) was a shambolic, inefficient mess--a point that Ian Kershaw makes with especially emphasis but is distributed across much of the scholarship by historians who have studied the Nazi era.
So many other periods of human history are not used as common metaphors or if they are people are not very curious about why (e.g., why should we speak of "Byzantine politics"? There is a very good reason but I don't think this rises to a conscious thought for many people in everyday life.)