You rarely find hawkers in developed countries. There are designated places to trade. That creates order and cleanliness. We could quickly interpret that as a sign of development. Everything is formal. 

In Kenya, every available space, including underpasses, overpasses, sidewalks and even the road itself are markets. We can blame poverty, poor regulation, and slow economic growth.

Many hawkers and small-scale traders would leave the streets if they made enough money or got better jobs.  Competition makes it hard to scale up, with new competitors joining the market every year. Some traders have specialised in selling one type of product.