But it seems its most common in india. I searched for 'africa vegetable market pics' and I found a few outdoor carts + hand baskets. And I doubt if it has to do with poverty as such.

https://www.shutterstock.com/search/african%2Bvegetable%2Bmarket?search_source=base_related_searches§ion=1&page=4

The last minute  possible reason popped into my mind: Could it be due to road condition? Its less likely mud and dirt got kicked up to head level than to say carts 1 foot above the ground or to carrier pole baskets at waist height; important if one's carrying food. Actually my impression is carrier poles are disppearing in china