Because Nigeria like most African countries is made up of a dozen ethnic groups and has a large Muslim population in the North to boot. Most Nigerian immigrants are Igbo who are Christian and have the highest IQ out of any other ethnic group in Africa. This is actually going to make Nigeria worse because the brain drain means all of the talented people are going to flee elsewhere.
“the brain drain (in Nigeria) means all of the talented people are going to flee elsewhere.”
That is a big factor in the Nigerian success rate here - we are disproportionally getting immigrants from the cream of the crop of a large nation.
It used to be like that from all over, and would be again, if we adopt the kind of merit-based immigration system that the Trump Administration is proposing.