Actually they weren’t. Nubia was building cathedrals with frescos when much of northern Europe was pushing berries up their noses.
https://en.wikipedia.org/wiki/Faras_Gallery_at_the_National_Museum_in_Warsaw
Christian Nubia was destroyed by Muslims.
Ethiopia also has some old and amazing architecture...look up the churches of Lalibela.
What happened to them that by the time Darwin got there they were reduced to savagery?