Blame the teachers who spread out the first three chapters (Columbus, Pilgrims, colonies... everything pre-revolution) in order to stretch through Columbus Day to Thanksgiving. Then you really quickly got the pre-revolution setup in December, the Revolutionary War and the Constitutional Convention in January, all the stuff about slavery in February for Black History Month, followed by the Civil War itself in March, Reconstruction to WWI in April, the Great Depression and Cold War in May, then by the time you're checked out in June you've got about three days to cover the Vietnam War.
I swear we'd spend months talking about colonies and who founded what and Oglethorpe and charters and other crap, then, like, 30 seconds on the stuff the teacher was actually alive for.