While I have never been to Pakistan, three main reasons come to mind based on what I have read about its soils:

• They tend to be low in sulfur. Ammonium sulfate is a cost-efficient way to supply this nutrient
• They tend to be calcareous. Ammonium sulfate helps solubilize phosphorus and micronutrients tied up with calcium
• They tend to be saline. Ammonium sulfate is a preferred source over urea on these soils

Ammonium sulfate, used within a fertilization program aiming to balance plant nutrition, seems like a good program to test against the current fertilizer practices in Pakistan.