In this post Dani Rodrik explains why trade theories suggest that the U.S. should liberalize trade for agricultural products (especially cotton and sugar) and abolish visa restrictions on on highly-skilled foreign workers. This will produce gains for the U.S. society as a whole and probably for the poorest part of the world population, that happens to be made up of farmers. (Of course India will probably see some of its engineers flee the country, but that is not exactly a win-win game).
But countries are not ruled by trade theories. Usually they are ruled by people seeking to keep power as long as possible. And sometimes people rely on minority groups within their society to keep themselves in power. Have a look at this paper, “The Diminishing Effect of Democracy in Diverse Societies” by Gilat Levy and Oriana Bandiera (London School of Economics and Political Science). Indeed, this can explain why western Europe heavily defends its farmers (4-5% of the population) sacrificing the common good.
An interesting theory should consider not how much a single group benefit or not from trade liberalization but how much influence the group affected by the new policy has on the decision making process. A reduction of trade barriers can help to tackle chronic poverty (have a look at the “Industrial Development Report” by the United Nations Industrial Development Organization). But barriers are not where they are because governments think they are irrational from a political (not economic) point of view.