Why aren't all US nationals also US citizens?
This article on Stilt explains some reasons why someone might be a US national without having US citizenship:
That is because there are individuals that are born in the Commonwealth of the Northern Mariana Islands, Swains Island, and American Samoa who are U.S. nationals. Both these places are possessed by the U.S., which is why people living there have U.S. national status and are protected by the U.S. government. Therefore, they are nationals but do not hold U.S. citizen status.
Apart from that, there are situations that can make you a U.S. national. If both of your parents were born in either Swains Island or American Samoa and they also lived in the U.S. before your birth, then you might be a U.S. national. Even having one parent that was born in any of these places and then lived in the U.S. would make it possible for you to be a U.S. national. In this situation, though, the parent would have to meet some specific requirements for residency.
This distinction also carries political implications. For example, non-citizen nationals are not allowed congressional representation. (Some) US territories have representatives in the US Congress, but they are not allowed to vote and when they move to the US they need to become citizens before they are allowed to vote in federal, state or local elections.
I'm wondering though, what is the more fundamental reason for drawing a distinction between citizens and nationals? Was there an explicit decision to make this distinction or did it just follow from the fact that the territories are not US states?