So as a person native who was raised on Hawaii it's part the Japanese influence, and all the other people who immigrated to Hawaii during when there was high demand for ... Sugar Cane Plantation jobs. Due to the colonization that happened there , not much of the native population was left to do it. So Japanese, Okinawan (some, I believe), Chinese, Filipino and Portuguese people were told to come .
Where the making of spam musubi comes in is that Plantation workers were not paid very much though needed food. And there was not many foods they used to have. Spam was cheap and plentiful though, So Spam musubi was created to fill that need.
Though this is just what i was told, and it seems the origins is disputed.
Heres an article about it-βΌοΈ CW Mentions of Internment camps (edited)