It's whay I called the "Old Railroad Theory"
The US has build their subway systems 100 years ago, but now doesn't want to take down and rebuild a new one.
Meanwhile, a country developing the subway recently would have the newest and lastest systems and technology, since why use old technology if you're starting from scratch.
The US have no platform safety barriers because they're used to it.
Meanwhile, China only recently has subway so they looked at the US and thought "Hmm, looks like people can fall on the tracks" so they built a platform safety door system.
Like imagine telling schools to get rid of all their windows XP machines. Old habits die hard.
TLDR: Its easy to use new technology when you are building new, vs having to dismandle the old system and then build it again.