Lack of federal income tax I believe
More seriously, is this something that's true of territories in general. I was reading the other day and was kinda disappointed I can't even visit most territories.
I know we can go to Guam, PR, what territories can't we visit?
Some of them have very good reasons, for example at least one was a nuclear test site. One of them is visitable, but you basically have to sail a private boat (I wonder if a rental is fine) from Hawaii to reach it and iirc need to get permission ahead.