I think a combo of things, but dietary is the lead cause. Indigenous cultures all have low rates of chronic disease, until they are exposed to the SAD diet. Throw all the rubbish, chemical additives, fake foods and high glucose foods both simple sugars and starch and you get the result you see.
I think the high rates in island and small populations compared to the usa's level is because those small populations are so much more easily flooded with rubbish imported goods. Which changes their way of eating very quickly. It's also a lot easier to do the numbers on small populations compared to a 300 million plus country.