That trend is not really accurate. Museum displays often have a selection bias in favor of masks and sculptures, while ignoring many other types of african pictoral and textile art.

For instance, there is widespread tradition of decorative figures put on building walls. From lunda wall paintings in Angola to Hausa tubali architecture in Nigeria, or church murals in Ethiopia. There are even older traditions of rock art that spans the continent. For obvious reasons, it is difficult to take entire walls or rock faces for display in museums. Photographs can be taken of these works, but there is a difference between an image of an object and the object itself.

Also, there is a whole range of fabric art, from Kente cloth of Ghana to Kuba cloth from DRC. Actually, this page gives a good and picture-rich overview of several of the different fabric arts.

There is also the whole genre of Asafo flags from Ghana. Although, that genre represents melding already-existing fabric art with colonial influence (e.g. the Union Jack). So, perhaps not strictly "traditional"

Or drawing on leather like that Hausa mat. That particular example is 20th century, but represents continuity with earlier islamic art patterns. Similar to the Asafo flags, Hausa leatherwork artisans could adapt their form to represent scenes of colonial life

---

Now, on the other hand there is a whole discussion to be had about the politics of colonialism and museum displays, and the concept of "traditional art".

To put it bluntly, a lot of the most famous traditional african art on display at major western museums like the British Museum, Quai Branly, Victoria and Albert, etc; were taken during the colonial era. In the museum world, there have been some bitter debates about whether artifacts in those museums like the benin bronzes Emperor Tewodros' crown or numerous sculptures and masks in the Quai Branly should be considered looted art during the colonial era, and whether western museums should loan the objects back or permanently repatriate the objects.

This becomes relevant to discussion of traditional art because, the taking of art from Africa for study and display in Europe and North America during the late 19th century did much to set the agenda for what constituted "traditional african art". Attitudes by 19th century anthropologists and art historians that since kente cloth or beaded objects were worn on the body, and were largely geometric designs rather than representations of humans or animals, they did not resemble "fine art". Instead, objects like the Benin bronzes, Masks, or Nkisi figures, were placed in museums where they could be displayed alongside Greco-Roman bronze and marble sculpture. To an extent, this was comparing like-with-like, since these museums tended not to include European clothing or costume in displays of "fine art".

But, on the other hand, the display of nkisi figures, masks, thrones served an ethnographic and anthropological narrative. Nkisi figures and Melanesian dayak sculptures were collected by colonial ethnographers. Ethnography/anthropology in the late-19th and early 20th century was deeply wedded to a paradigm that European culture and religion was more "evolved" or "civilized" than the rest of the world. One of the major intellectual currents in justifying colonialism was the "civilizing mission" which sought to remake "primitive societies" and make them more like Europe.

So, the sending of masks, sculptures by ethnographers to museums in the European metropole were both intended for viewing by the European population as proof of this "primitivism" and justifying the civilizing mission. But also, intended for study by ethnographers and colonial officials, as a way to "know the primitive mind" by studying objects that were both artistic and had religious/ritual significance. That also reflected judgments that cloth was merely decorative and could not reveal African mindset. Also, as I said above, the portability of an object was a concern to the ethnographers, encouraging them to take pictures, but preventing the shipping of murals.

---

So, why has that understanding of "traditional" African art continued to the present in museums?

There are a few factors. Museums already have these sculptures and masks in their collections, and it's easier to display what you already own.

The definition of "traditional" African art as objects not showing outside influence raises complications of categorization. Does art from areas influenced by Islam count as traditional, or is it better categorized as Islamic art? Ditto for Ethiopian christian art.

Also, there is the question of preservation and durability of materials. I talked a lot about fabric and leather objects above. Unless care is taken to preserve fabric and leather, those objects can quickly degrade in humid tropical climates. So, art historians in the late 20th/21st century travelling to Africa for early 19th century fabric and leather may have a hard time finding it.

Also, there is the controversy over repatriation of art, and questions of cultural heritage in post-colonial African countries. Because a lot of art was looted in the colonial era, there has been movement towards a new regime of ethics in acquisition of artifacts. Countries like Nigeria are very careful about letting any more pre-colonial and colonial era art go overseas without strong safeguards that it will return. (The story is a bit different for modern African art)