Four of the most prolific serial killers were based in south American countries like Colombia, Ecuador, Peru, Venezuela, Brazil etc.
After those, it’s India, China, Pakistan, former Soviet states, Russia and so on.
Hardly first world countries, and especially not when these killings took place.
Those killers’ body counts eclipse all American killers’ by a very significant margin.
Gary Ridgway (the Green River Killer) is the most prominent American serial killer by a mile, but he’s not that “famous” even though he was arrested and convicted rather recently (about 20 years ago).
He is an uninteresting person who is rather stupid, doesn’t do interviews and doesn’t revel in his infamy like the more famous serial killers do or did. He’s almost more akin to a rabid animal as far as public perception goes.
Furthermore, the decline of serial killers, even with more interest and attention than ever, I think is largely attributed to better social safety nets.
As it turns out, it seems that in at least some cases, socialistic policies work surprisingly well.
Wasn’t a serial killer with the arguably largest kill-count a doctor from the UK? I can’t remember his name, but it’s not just an American thing.
There was also that dude in Argentina (?) who had hundreds of kills attributed to him, mostly children? There’s the Chess Board Killer as well, he was Russian and was trying to kill a person for every square on a chess board. (I believe he was a few short before he was finally caught).