More can always be said, but these older links may be of some interest:
https://www.reddit.com/r/AskHistorians/comments/2gkumo/why_did_alaska_and_hawaii_become_states_but_the/
https://www.reddit.com/r/AskHistorians/comments/439n5c/why_is_hawaii_a_us_state_and_not_say_guam_or/
I wrote a post about it here. There's always more that can be said.