A lot of historical fictionauthors - especially the big names - are British. By and large I think it is fair to say that a British author is more than likely going to write about something European related than they are something American related. That's probably a by product of Europeans generally having a greater appreciation of their history than Americans (just my opinion). That, and it's probably more profitable for the publisher as there are far more events in European history than American so they promote it as such.
There's also a certain romanticism to European history that all the other continents lack, partly because ours (I say ours because I've always considered myself European) is documented for so many centuries and has had such a volume of work already written. American history has no King Arthur figure (of which a
lot of fiction is modeled on), no Caesar, no Alexander the Great, no Henry VIII and no Napoleon. Your major historical figures aren't so well known outside of Washington and Lincoln. Everyone knows who Caesar and Napoleon are whereas 90% of your presidents are in-and-out so quickly they barely leave a mark on the rest of the world.
Myself, I don't find American history particularly interesting. I'm a total Europhile when it comes to history. But another thing you've got to remember is that there are plenty of people in this world that rather rightly or wrongly dislike America and Americans. You don't have a particularly great reputation around the world. The thought of being forced into reading something about America would piss plenty of people off simply because it's about America. Of course, that's ridiculous and narrow minded. But there are a lot of people that simply dislike America and Americans to the point where many books about them/it would be unprofitable for publishers outside the US, so they don't bother, and because so many authors of historical fiction aren't American there's not as much demand for it.
I hope that made some sense.
