After the exploitation of the New World and the Fast East, European powers were constantly looking for new lands for resources and markets. However, this was publicly backed by the idea to spread Christianity to 'savage' people.
Most of Africa followed local religious beliefs and these were use as a pretext for the 'White Man's Burden' to spread Christianity and enslave these people.
Africa at the time had many warring factions and due to the disunity on the continent, Europeans were able to colonize it quickly.