It was hard for THEM to understand. They'd never been enslaved by anyone, other than the usual drag it is to have to make a living. Being outstanding in every way, they'd not had any especially hard life - high IQ, super talented...and so on. They did fine, personally, as did we all. That was what made that weird, for them and for us. Outside the middle east, most people don't give a crap about what ancestors did. At least these guys (and gal) appeared not to.
Uh... that is simply not true. There is still tremendous animosity between the people of the American North and the South, because of the Civil War, which had the largest death toll (in proportion to the population) of any war in history. There has been so much migration in the last century and a half, that a large portion of the people who live in those places are not descended from ancestors who lived there (or even descended from ancestors who lived in other countries). But there are enough people who are the great-grandchildren of Americans who died in the Civil War, that they still think of each other as "Damn Yankees" and "Rednecks" or "Hillbillies" or several other epithets.
Not to mention, despite the civil rights movement that was (arguably) launched by President Truman in the 1940s (when he integrated the armed services) but attained a lot more momentum under President Eisenhower in the 1950s (when he integrated the nation's schools), and finally became a
cause celebre in the "counterculture" of the 1960s, neither Afro-Americans nor their sympathetic Euro-American friends have forgiven the many generations of Americans who enslaved them, then passed the Jim Crow legislation after the Civil War, then continued to discriminate against them in housing, employment and public life up through the 1950s, and continue to shoot them every chance they get... (stop for breath) there is still tremendous animosity between our Afro-American community and a large segment of the Euro-American community, especially in the South and other Republican Party strongholds.
As for other parts of the globe, ask the average older Frenchman, Belgian, Czech, Pole, Dutchman, Dane, etc. how he feels about the Germans.
The Chinese are still pissed off at the Japanese.
Oh, and don't get me started on the Native Americans!