The explanation is that while the Malmedy Massacre was certainly a very notable event that occurred during the Battle of the Bulge, it wasn't something that directly affected the outcome of the war. As such, teaching it to high school history students isn't deemed necessary in order for them to obtain a basic understanding of why America was fighting in the War and what effects that had on America as a whole, both during the war and as time went on.

The unfortunate thing is, while many notable events like the Malmedy Massacre occurred during the war, it becomes incredibly difficult to teach the overall history of the war to high school students if you suddenly start including every notable event that occurred.

Sure, some teachers may decide to touch upon that particular topic, but if it were suddenly required that every high school history course cover the Malmedy Massacre, the question then becomes what event or topic gets dropped in order to cover this relatively small (but still notable) event that doesn't really affect the outcome of the war nor the layman's understanding of it.