Why did trading empires develop in ancient West Africa?

Going all the way back to 1500 BC, West African civilizations were actually highly developed relative to other ancient cultures—they were developing iron smelting and other technologies better than most. Because of this, they and their descendants held control over trade platforms because they had valuable things to contribute. This was exaggerated in the seventeenth and eighteenth centuries, when African colonization came through and the slave trade began to supply them even more trading opportunities. These powerful empires, who had established strength with their advanced technology and access to water routes (the Atlantic Ocean to travel along the coast), engaged with the European slave traders and would go as envoys to collect slaves from inner nations, transporting them to the coast, where they would be sold and loaded on ships because of their access to the sea.
Technology advancements and waterway access gave them advantages over nearby nations, which helped them grow strong.