If developing countries could only sell more export crops they could afford to buy more food.

Export crops do not feed people. Why grow one crop to sell to someone else so that you can buy food, when the land you farm could produce the food you need? The prices of export crops are not controlled by the producers, and tend to remain low, so that it is almost impossible to raise all the needed revenue. And even then, it is a question of how that revenue is divided. Who gets the biggest share?