I imagine that are many reasons that people cannot understand the horror of what happened in World War II and actually, previous to World War II. But a few ideas do come to mind immediately. First, while other genocides have occurred subsequent to the Holocaust, none has been on the scale that the Holocaust was, and I honestly think we cannot wrap our minds around the enormity of it, the fact that millions of people were systematically put to death largely on the basis of religion or ethnicity. Second, we all think of ourselves as good people, and we find it hard to understand how ordinary people, presumptively good people, could allow such atrocities to happen, a lack of insight into those people in those times and places.
Just looking at the number of Jews killed, we find our minds reeling, six million Jews just gone. In Rwanda, 800,000 Tutsis were put to death, and even in Cambodia, which stands out amongst genocides, the number is about two million. If you have ever been to New York City, you might be able to imagine three-fourths of it wiped out. But that is still difficult for any of us to conceptualize. Hitler ultimately approached all of this in an exercise in efficiency, experimenting with means of killing that would not cost him overly much and that could be done quickly. This scale and approach of man's inhumanity to man should be difficult for us to understand. I would be quite concerned about anyone who said, "Sure. I can see how that could happen."
Similarly, we do not want to be people who have mindsets that allow us to visualize being part of a situation like this, either actively helping to slaughter a people or simply standing by saying nothing. No one wants to think him or herself capable of this. It is like a defense mechanism we all have, that keeps us our consciences clean. We tell ourselves we wouldn't let this happen. Sadly, this is not true. We are all capable of allowing this to happen. One famous study showed that ordinary and decent people were perfectly capable of shocking another person, even to a life-endangering point if ordered to do so in an authoritative way. This was the Milgram study. We are all quite capable of this. That is, in fact, the point of Shirley Jackson's "The Lottery." The towns around the concentration camps smelled people burning. They saw people crowding the rail cars, going in but never coming back out. But denial is a powerful force, and as long as we can tell ourselves we simply cannot understand this, we believe it can't happen here.
Unfortunately, our refusal to understand could doom us to other holocausts. The situation that led to the slaughter of Jews and Gypsies and many other groups was a situation in which things were not going well in the economy and stirring up the populace to blame the problems on some "other" yielded Hitler, the Nazis, and the Holocaust, even aside from all the others who died in World War II. There are parallels today, in Europe and the United States, and history should not go unheeded. Not understanding is something we cannot afford.