The idea of national treasure, like national epics and national anthems, is part of the language of romantic nationalism, which arose in the late 18th century and 19th centuries. Nationalism is an ideology that supports the nation as the fundamental unit of human social life, which includes shared language, values, and culture. Thus national treasure, part of the ideology of nationalism, is shared culture. A national treasure can be a shared cultural asset, which may or may not have monetary value; for example, a skilled banjo player would be a Living National Treasure.
https://en.wikipedia.org/wiki/National_treasure