It showed up, because people who had been in effected areas, went to one of them on vacation, got bitten, and brought the disease back when they came here. They were then bitten by American ones and so the disease was spread.
Add one more detail onto your list....communities in the US are doing anywhere from an effective job to a lousy job in mosquito control. When I stationed around Shreveport in the 1980s...they sprayed weekly in neighborhoods and you just didn’t have any mosquito issues within the city limits at all. I would be asking about these Florida communities and what they were doing before all this hype?