Well, I would ascribe it two two basic factors.

First, there is very limited written record in Sub-Saharan Africa prior to European contact. So, when a potter makes some Urewe ware with a geometric pattern based upon a mathematical pattern they have in their head, scholars many centuries later might appreciate the pattern but not spare much thought to what sort of mathematics a person would know to make the pattern. Whereas, because Greeks wrote it down, we know that they had a concept of the Golden Spiral, for example.

Secondly, standard treatments of the history of mathematics do reflect a Eurocentric bias. To quote Helaine Selin Leigh N. Wood:

Mathematics has been part of all societies, a part of every profession as well as everyday life. Western Mathematics became narrower with the insistence that only deductive mathematics from a fixed set of axioms, following the Greek tradition, was real mathematics.

or Ngomotso Garegae

It is a known fact that mathematical ideas exist in any culture, regardless of its level of primitiveness. These concepts came into being as a people's response to their survival needs of ordering, numbering, measuring, counting and constructing. .... In most African societies, the traditional notion of mathematical concepts may have been "tinted" by the western world through the introduction of Christianity and missionary schools. For instance, cultural initiation to adulthood which used concepts of measurement was discouraged through Christian beliefs....

Fortunately, there has been much work done in the fields of Anthropology and among professional mathematicians since the 1970s to expose the history of forms of math from around the world.

Claudia Zaslavsky's book Africa Counts: Number and Pattern in African Culture, written in 1973, was an early exploration of this topic with respect to Africa specifically.

More recently, the Mozambican mathematician Paulus Gerdes has written extensively on topics of geometric pattern in African traditional art and daily life. His book Geometry from Africa: Mathematical and Educational Explorations is educational both from a cultural/historical as well as mathematical perspective.

You could also check out Mathematics Across Cultures edited by Helaine Selin, which contains that first quote.

Or you could read What Mathematics from Africa which contains chapters contributed from many African mathematicians, and is the source of my second quote.