Why aren't all US nationals also US citizens?

Question

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?

Answer 1

The distinction was more or less invented or at least cemented by the US Supreme court around 1900:

Under the Insular Cases of 1901, the Supreme Court ruled that unincorporated territories and insular possessions of the United States, which were not on a path toward statehood, had limited applicability of the U.S. Constitution. At the time, these included Guam, the Philippines, and Puerto Rico, acquired in 1898 at the end of the Spanish–American War. According to the decision, those born in insular possessions or unincorporated territories were not eligible for citizenship, though they were considered nationals and could hold a U.S. passport and gain diplomatic protection from the United States.

The question was brought about in the aftermath of the Spanish-American war:

When the war ended in 1898, the United States had to answer the question of whether or not people in newly acquired territories were citizens, a question the country had never faced before. [...]

The Court also established the doctrine of territorial incorporation, under which the Constitution applied fully only in incorporated territories such as Alaska and Hawaii. Incorporated territories are those that the U.S. Federal Government deems on a path to statehood. Meanwhile, the Supreme Court ruled the Constitution applied only partially in the newly unincorporated Puerto Rico, Guam and the Philippines.

According to some scholars cited by Wikipedia, this was basically a form of colonialism.

The aftermath is rather complicated to fully discuss here, but e.g. Puerto Ricans were eventually grated citizenship, while the Philipines was granted independence. American Samoa is probably the most sizable exception in that it remained "in limbo" in this regard. As recently as 2015 US appeal courts upheld the Insular decision(s) in that regard, with the US Supreme Court declining to revisit.