Probably because the Norse migration into the lands now known as France is not emphasized in American schools as much as the later invasion of England by the descendants of those Norse who settled in what was called Normandie: The land of the Northern Men. Norse-Man plus the suffix "die."
It's my opinion that the history of western Europe from the time of the fall of Rome till the "renaissance" is poorly taught. AND, many children's history books leave the children with the impression that places like Spain and France were single countries hundreds of years ago. France was for about 200 years but was split up in the mid-800s.
In many places, the idea of a "country" as we use the term today didn't really exist and instead loyalty was given to a family and its head. Thus many of the fuedal wars were not seen as treason against a land but rather as the result of the death or imprisonment of a liege lord.
But my point is that much of what is taught to children is "simplified" to make it easier for them to grasp. While the reality of the time periods was often much more complex.