The salt is a mineral that is of great biological importance for the people. Not just that it is important, it also makes the food taste much better when added in it. This made the salt very popular and demanded among pretty much every civilization or tribe, meaning that the ones that were controlling the trade with the salt were able to have a lot of profit of it. The kingdoms of northern Africa had that chance and they used it. They were in excellent location to control the trade routes through which the salt was passing and was traded, so they were able to get a lot of wealth even though they were not the main producers of it.
