For one, Jamaica has just 2.8 million or so people, the USA has around 314 million — that means the US has more than a hundred times more people to find their sprinters in. GDP per capita is $9,100 in Jamaica, and it's $49,000 in the USA — that's five times the money to buy starter pistols, studded boots and Gatorade.
What is behind Jamaica's remarkable athletic success?
— Why Jamaica Is So Fast - Business Insider
I think you'll find it's a lot simpler than this: Jamaica, like the rest of the Carribean islands which had athletes in the 100m heats, is so small that being a long distance runner would be boring. You're run right round the island every time you popped out for a "quick run" .