Most of Sub-Saharan Africa was Francophone, and that's represented in African immigration data as well. The US might be able to attract some amount of brain drain from Nigeria, but it's much less likely given the changes in immigration policy in the US over the past 10 years. A Nigerian who may have been brought on an H1B will now most likely be brought by an employer to Canada or the UK, or a GCC will be formed within Nigeria.
Furthermore, the question is what do you define as "African". Africa is a massive and diverse continent with various different countries with varying levels of state and human capital capacity. The kind of immigrant coming from Nigeria will be significantly different from similarly sized DRC.
Also, as a 1.5 gen immigrant (I immigrated as a toddler), it's easier to immigrate to much of Western Europe and the UK compared to the US.