Well there's a very long list of reasons starting from geographic isolation to brain drain (incl. to the USA) all the way up to military coups, but I doubt that was a serious question so there's no point me answering your question in any great detail.
> What makes you think it's normal for Fiji to have low standards of development, but it's an aberration for the USA?
See above for why Fiji has low standards of development. In comparison, USA is the most prosperous and powerful nation on earth and has been for over a century. You've enjoyed democracy for almost 250 years. You've been to the moon.
> the US ranks poorly on some of those dimensions, due to extremely complicated and maybe intractable political and cultural issues.
And at last after the defensive stuff is out of the way, we see an attempt at an answer, abstract and hand-wavy though it is.