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 European and North American contexts, 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 agree with /u/Crrpit's answer that it is fundamentally about familiarity, and the fact that WW2 and Romans is a part of the school curriculum in US, Canada, UK, Australia (where vast majority of user base is from) is a large part of it.

I'd just extend it to bring up the topic of media and popular culture.

If you asked me to name recent movies or TV shows or video games that deal with Nazis or WW2, I could immediately name some. Operation Valkyrie, Band of Brothers, Hearts of Iron, Inglorious Basterds, Fury, Man in the High Castle, Wolfenstein.

Ditto, for Rome, there are movies like Gladiator, Spartacus. Plays like Shakespeare's Julius Caesar. There is a whole franchise built around Rome Total War. Assassins Creed Origins was set in Roman Egypt.

So, even if folks are not encountering those subjects in school, they are still very likely to encounter them through cinema, television (and streaming services), video games, or even books. Rome and Nazi Germany are staples of mass-market history and historical fiction.

Pop culture also drives questions in other ways. When the movie the Siege of Jadotville came out on Netflix, there were several questions asking about it's historical accuracy. When Mandela came out (coinciding with Nelson Mandela's death) there were many many questions about his life and the struggle over Apartheid in South Africa. There are regularly questions that reference the movie Django Unchained.

Rome and Nazism are just part of our users media diets.

In my opinion it is because of their appeal to the sense of curiosity concerning alternative history, and their respective associations with vast empires and large conflicts. The Nazis also provoke a strong mental association with the Holocaust as well, which, IMO provokes a high degree of reflection and moral reaction among most people.