Himler was fascinated by the ancient germans and sent SS "archaeologists" to search for the traces of the ancient aryans. But Hitler responded that they only dig up clay pot, and associated them with the eating of raw meat. For Hitler the "real"ancestors, the models are the romans and the greeks, not the celts. That's why Athena Parthenos was leading a parade about the evolution of the german art. This view came from the french Gobineau, who was the first to write a link between greatness of civilizations and the white race, with reasoning like, in the old empire the egyptians built pyramids, so they were ruled by white (arian) leaders, and they were unable to do it afterward. Of course it's a bit far from the celts, but the main idea is that no great celtic monument survived in comparison to mainly Greece and Rome, so they were not appreciated by Hitler who liked greco roman monuments. That is of course, only the big picture.