African american are more succesful in most sports, than most other racial groups, not only boxing.
The success in boxing comes from a mixture of genetic ability, such as large amounts of naturally occuring fast twitch muscle and possibly generally stonger chins. Also, environmental factors such as large amounts of poverty in the black community in America creates a primitive hunger to suceed at all costs that many white middle class colledge kids simply dont have. This hunger is more important than anything else in boxing I think. For the same reason many decent gyms are in largely black areas, the trainers and other boxers are black and there is a black style of boxing in America, which centres on slick counter punching compared to the mexican/irish style of coming forward with your hands up. Which is why many great fights are between mexican and black fighters, because styles make fights.
You cant generalise people of an entire continent. African Americans are almost exclusively descended from the slave trade of people from the west coast of Africa, mainly countries such as Nigeria. A Jamaican and a Nigerian will generally be quite similar physically. East coast africans are again very different physically as are south africans, of course these are only sub saharan africans, africans from above the sahara such as morocco, egypt, or tunisia are again very different and more abaric / south european in appearance. So only really a west african is similar to african american.
"African-Americans are good only in sports that requires physical advantage because they are big and strong...That's their main asset...You can rarely find an African-American in sports that requires a great deal of thinking like billiards, poll, chess, and other sports that doesn't require physical advantage but rather superior thinking..."
Well I think boxing requires superior thinking. Even Teddy Atlas says on fight night 4 says the winner of a fight is usually the smarter of the two in boxing! :D But I doubt typical afro american environment and genetic ability is suited to a mind sport such as chess.