It's not about the plot, it's about the advancement of the setting The events of Fallout 1 did effect Fallout 2's setting. And New Vegas showed us much of what followed from that, the NCR expansion, the Enclave becoming a small group of retirees. Even Black Mountain and that old ski lodge are showing a facet of the aftermath of the Master's army.
Oh, sorry I misunderstood. Still you could have all those elements on Cuba, since the Platt amendment gave the US the right to set military bases on the island. It makes a lot of sense that the US government would outsource their morally questionable research to the third world, oh wait..they did.