Diseases and weather. Western Africa was known back in the day as "The white man's graveyard", and it's just training wheels compared to Central Africa. In high school we learned about the many early missionaries who were sent to preach in rural West Africa in the 1800s. Like 90% died within 6months to 2 years even though like 30% of them were mullatoes. Also most of the continent is non-navigable by waterways so you can't sail into the interior.