Assuming this isn’t just a big psyop trying to scare US adversaries, and that UFO’s are real… the US was the first to detonate nukes in 1945, and on its own territory, which might explain some of it. Maybe the UFO’s were investigating how scientifically and technologically advanced humanity was becoming, and naturally focused on nuclear tech as a leading indicator.
The 1936 Berlin Olympics TV broadcast might also have alerted them to humanity, and travel time of the broadcast plus travel time of UFO’s to Earth (assuming they have lightspeed capability) might have resulted in them arriving right around when nukes were invented (if they’re from, say, Alpha Centauri which is 4 light-years away).
The US and Europe also have free press where things like this are more likely to be publicly reported than in say, the USSR back in the day, or China under the CCP. Which in conjunction with the above may explain sightings maps like this one, where sightings are most frequent in the US and secondarily in Europe:
https://updb.app/map?zoom=1.00&lon=149.0917&lat=37.3003
Also, if you search number of reports by date ranges, look at what you get:
1-1943: 1566 sightings reports
1944-2023: 294,641 sightings reports
Two orders of magnitude more sightings reports in the 80yrs since nukes were invented, vs in the almost two millennia previously. Obviously we also had more robust and technologically advanced media in the latter time frame, so that contributes much to the discrepancy. But still a huge discrepancy.