Jamaica appreciates track and field more than America, it's that simple. You've been to Champs yourself Rojo, so you've seen first hand how much they love the sport and how something as mundane as a HS track meet has become like an annual festival.
They follow it constantly, as opposed to America who only pay attention every leap year, and their sprinters are the most revered athletes in Jamaica, more so than the cricket team or footballers. And as you've shown, the Jamaican national media have already begun to embrace her as potentially their next great track star. Now tell me, would she be getting the same attention in the US as an American sprinter? Absolutely not!
And it's not just Briana: Chantz Sawyers, bronze in the men's 400m, was born in North Carolina. Damion Thomas, the 110mh champion, is from Florida (whether he was born there or not, I don't know).
Being Americans, these three athletes would've represented USA by default. That they went out of their way to switch and have explicitly expressed pride in doing so suggests that they were personal decisions, not professional, and not one they'll be going back on for now. Whether their endorsements could be larger as American athletes is debatable; I'd have to see some data on that front before forming an opinion.
One more thing: I've always had the opinion that if Usain Bolt was American, the world would've hated him because his laid-back attitude would've been seen as stereotypical American arrogance instead of the bravado and partying mindset that Jamaica is known for. With this in mind, perhaps being of foreign (and particularly Jamaican) parentage has enlightened these young athletes as to:
a) how the rest of the world sees Jamaican athletes, especially Usain Bolt,
b) how the rest of the world sees the United States in general, especially after electing you-know-who.