And ~18 years later, the crime rate started dropping. Everywhere. And crime rates are generally still dropping.
Lead is a known neurotoxin that interferes with people's executive functioning, which is the part of the brain/consciousness that weighs decisions before acting.
Turns out that when you stop dumping lead into the brains of every single person in your country, their executive functions improve. Instead of impulsively doing stupid shit, they think about it at least a little bit more.
Crime and lead exposure has been brought up multiple times here. There's no better explanation, including "legalized abortion", to demonstrate it.
You can look at different countries banning leaded gasoline and those countries crime rates across the world, and watch as 18 years later crimes rates fall off a cliff, unrelated to the US legalizing abortion in 1973.