Why are American Sāmoans not U.S. citizens?

American Sāmoa has been a territory of the United States since 1900. However, those born here are not considered U.S. citizens but hold a special status called "United States national". 

Although all U.S. citizens are considered U.S. nationals, not all U.S. nationals are U.S. citizens. Those born in American Sāmoa - if they are not eligible to be citizens through their parents - are the only residents of any U.S. territory who are designated with this legal status.