Probably because decades ago the leftist controlled public school systems switched from teaching history to ideologically and politically indoctrinating students with cherry-picked sections aimed at 'interpreting' what really happened . . . the leftist way.

yes, it began in the 60's in US schools

Or with the creation of the Dept of Education.

MR ROGERS. The peanut man pres. did lot more damage than people know Department of Education Organization Act, which President Jimmy Carter signed into law on October 17, 1979