I think it's probably a mix of not being the subject area of the timeline (which, fair, everyone can't know about everywhere all the time) and This Is Your Brain On Presidentialism. I know it took me a while to wrap my head around how parliamentary struggles can be interesting, actually, and if you look at the drop-off in American interest from, say, the Tudors and Stuarts to the Hanoverians, it's pretty clear that a lot of people are more keen on the whole "uneasy lies the head that wears the crown" thing than the South Sea Bubble.