Those places you just named have always had a lot of boxers in them. But the fact remains, dudes from all around the North-East region of America would head to New York to sign up and have access to the best gyms and trainers in the world.
And how much of a difference is the number of Puerto Rican boxers going to make anyway? It's a small island, not China.
Anyway, I have a source. If you hadn't noticed you can click on that statistic and it'll direct you to it, here it is again.