One misapprehension is clear, at least, from your statement that HS has "less arbitrary moderation policies". I see no evidence of any policies at all. I can't even see who moderates HS. HS is maximally arbitrary, not less arbitrary.

Aside from that, one obvious difference is the amount of traffic. Take for example questions on "ancient history": they're pretty common on AskHistorians (though the number of questions that I personally have been able to engage with has dropped drastically in the last few months), but on HS there have been only 9 questions in that area in the last three months. Less traffic means much less need for moderation. I remember when AskHistorians was pretty small (20,000 subscribers) and there were not many rules in place at that time. At the moment, HS doesn't have even half that many users; AskHistorians has grown by more than 2000% since then. It's not very surprising that practices have to be different.

A second difference is that AskHistorians is in the format of a discussion forum, while HS is purely Q&A and more wiki-like. HS also seems to frown on duplicate questions (though this is not stated anywhere: no moderation policies, remember), even though old questions can be closed to new answers. AskHistorians, by contrast, actively encourages visitors to repeat questions. Because of this kind of thing, the kinds of exchanges on the two forums are very different.

And of course the biggest difference lies in who visits the forum. To get to HS you have to choose to visit that specific site. AskHistorians, by contrast, is located on a very popular website that strongly encourages dissemination between forums, which means it's exposed to a much broader audience. That has upsides and downsides. One upside is that people like me get to see what people are actually interested in. The most obvious downside is that the website as a whole is justly infamous for juvenile bigotry and vile harassment of all kinds, and close moderation is obviously necessary to prevent that from swamping the historical content. I imagine that isn't quite as urgent on a site that has a natural layer of insulation.