Chinatown is a product of immigration. That's where non English speaking chinese people are absorbed into America. It offers the greatest means of support for immigrants who do not speak the language and are unfamiliar with the culture. They pool their money because that's the only choice. As they (their kids) learn the language and culture, they invariably move to the suburbs and strike out on their own. The longer the family has been in America, the less likely they're in Chinatown.
So obviously language is a big determinant in whether an ethnic enclave develops. Most Nigerians were brought up under the British system so even though they hang out together when they get here, they're not nearly as concentrated as alot of Asian immigrants because they don't need quite as much support.
Going back the Chinatown example, another determinant is class. Chinatown isn't full of software engineers, CPAs, and doctors. It's full of cooks, laborers and gusy selling Chanel socks. These people often don't have the means to support themselves so again, they congregate.
The one difference between Chinese immigrants and African Americans is that the chinese immigrants focus on shops and restauratants (btw, never ask about their financing), while 75% the business plans (if I see a business plan) from black people I know are record labels or clubs (you don't wanna know who's financing these either).