Because it's distinct culturally and politically from the 50 states and (along with the US's other territories) therefore not always necessarily included when one is discussing the United States. Nor do its inhabitants necessarily identify as Americans.
I'm not sure why you're being so pedantic, anyways. I was just hinting at a reason for why the author of this article didn't mention Puerto Rico when they said that Hawaii was the only place in the US where serious coffee cultivation occurs.
No, obviously not just like Washington DC. I'm not even expressing my opinion in the matter I'm just explaining why Puerto Rico isn't always included when people say "the US"! Why did I get downvoted?