With the salve trade, the Europeans made some of the stronger African tribes along the coast heavily dependent on them. That was largely due to the profit and weapons that these tribes were getting from selling slaves to the Europeans.
The slave trade and the slavery though got abolished, thus the profit and weapons that the Africans were getting stopped. That resulted in the weakening, and after several incidents with the Europeans, the Europeans attacked them one by one and managed to conquer and colonize them easily.