It's a study of young adults, aged 18-39. The results are to be expected. It's not a result of anything insidious. It is a natural process. 50 years from now, when all of us who either knew someone who lived through the Holocaust or lived through it ourselves are gone, people are going to care about the Holocaust about the same as they care about the Mongols. It's different when you can see the loss in your loved one's eyes or hear your teacher's voice quiver because she went through it, and when you just have to read it in a book, or watch a show about it. How many of us, after all, get all worked up about Vercingetorix and his people?
Ask that same crowd how WW1 started and I'll bet the results double if not triple. Once it's out of living memory, it fades away from all but the interested, no matter how horrifying. That's why games like civ are so important... They get more interested...