It doesn't exactly answer your question, but this recent META thread talks about why these same subjects seem to dominate the subreddit, and to a large extent internet discussions of history.

Why questions about ancient Romans and Nazism are so popular in r/AskHistorians?

/u/georgy_k_zhukov has previously answered a question about why we love WW2.

/u/commiespaceinvader has previously written about why academic historians tend to pooh-pooh the efforts of military historians.