David Friedman explains in a post titled “Why Improving Things is Hard“:
There are good economic arguments to show that we would be better off if we went to complete free trade. That seems puzzling—if we would be, why don’t we?
The answer is provided by public choice theory, the branch of economics that deals with the workings of the political market. A tariff makes the inhabitants of the country that imposes it worse off but the politicians who pass the tariff better off, since it benefits a concentrated interest group at the cost of dispersed interest groups. More concentrated interest groups are better able to pay politicians to do things for them.Trade policy is optimized, but for the wrong objective.