It's not hard to see the convergence...
Early American boxers from disadvantaged migrant backgrounds...
African American boxers from disadvantaged backgrounds once they got more a chance...
Mexican boxers... etc etc
Eastern European boxers from...
And yes you can argue individuals came from this or that- but the broad spectrum of society & opportunity (and I'll get back to that) creates the environment. How many kids who have a real shot at an easy ride to college because their parents read & were educated & had middle class jobs... how many choose boxing, even if they are good? Fucking zero point one %.
But kids who have fewer other role models, or choices or even spaces to go that they get attention at- they end up in a boxing gym. Or worse awaits them. And they follow through because other avenues are unrealistic.
You can pretend it's race but that's stupid. It's to a large degree economic, to some degree opportunity and to some degree knowledge- did you land in a place with enough boxing nous to move you forward. USA, Mexico have that in some areas, Soviet states & Cuba developed it deliberately through govt, anywhere else you're in pot luck of finding a good gym or overcoming not having one to start. They had a really thorough amateur coaching apparatus you can't begin to fathom in USA. They used to test every school kid for potential. That stopped but the hundreds of thousands of paid coaches at city & state level- literally a good paid govt job, they remained. And they still bear that fruit.
50% economics to make getting punched in the head worthwhile. 40% knowledge base. 10% variables.