I don't know if it's about controversy though. Sometimes it just takes society time to fully learn and understand what happened. A lot of the information society had about the Vietnam War during the 70s and 80s was incomplete until documents were released and eye witness accounts were told decades later. So I think text books and teachers shy away from recent subjects so they don't inadvertently teach something that turns out to be not true or leave out things that are. In the 70s and 80s Vietnam wasn't so much history as it was still the present.
The fascinating, and sometimes most frustrating thing about history is that there are really aren't "beginnings" or "endings" like it's taught in school or shown in movies. For education's sake I suppose, there are dates of beginning and ending generally assigned so when we are young we learn the basic Who, What and When. But as adults we usually learn that people and ideas don't just appear out of thin air and then are wiped clean. Vietnam both as a country and the troubles that caused America's involvement in a war there didn't start in the 60s and they didn't just disappear in the 70s even though the war had been declared over. The history of it is still being written and rewritten today depending on new information or eye witness accounts.
History is a continuum of interactions that involve human physiology, psychology, sociology, politics, economics, arts, religion, science, technology, and nature (which includes natural resources, phenomena, and disasters). Knowing that they interrelate and the general order of things is the answer to the the great WHY. Why we have the laws we have and why some are so controversial to different groups; why we have certain music/film/book/art genres and why they are so influential on different groups of people both for and against; why there are wars and why sometimes governments don't want to work toward peace; why some people are rich and some are poor and why the notion of equality in economics sounded so nice in theory but failed in practice; why economies thrive or fail; why there are famines and plagues and how they shaped the ethnic and economic make up of our world today; even why we have etiquette and why it isn't a static set of rules but changes over time. And when we know why we are who and what we are today -- we get a glimpse of what we will be in the future and can answer the next question of HOW to create it. Those that don't have any understanding of the past, can't understand the present, and can't help create the future.