Because acacia does so well in African drylands. Its also a nitrogen fixing tree which helps it grow fast as it does. The goal of reforestation in drylands is

Because acacia does so well in African drylands. Its also a nitrogen fixing tree which helps it grow fast as it does. The goal of reforestation in drylands is to increase shade to reduce evaporation. Once the acacia has provided enough quick shade they can (and hopefully will) plant other plants in the understory to increase diversity.