According to a report in public domain, "Nigerian has 80,000 registered Doctors. More than 50,000 are practicing abroad." Out of the remaining 30,000 doctors, "92% are considering finding a job abroad" while "70% ... are making plans to leave for foreign lands and are taking exams to that effect."
I am inclined to conclude that professionals, particularly the doctors, are deliberately being frustrated to flee Nigeria. This notion is reinforced by the recent news that procurement in the Ministry of Health must pass through Ministry of Agriculture.