Tariff and quotas are trade barriers that governments establish to protect national products. Tariffs are taxes imposed on imports and quotas are a limit on the quantity of a product that can be imported. These barriers are established when the government is willing to protect national producers when they are not able to compete with the low prices on the imported products. Also, the benefits of these restrictions are concentrated on the producers but its disadvantages affect all the consumers who have to buy products at a higher price. According to this, the answer is that tariffs and quotas are often imposed when a government is more responsive to producer interests, and the benefits of those trade restrictions are often concentrated.
This would be the opportunity cost: the cost on missing out on different opportunities once one opportunity is chosen. So if you choose A over B, the opportunity cost is the gain you could have had if you'd chosen B (even if you anyway couldn't have chosen both in the first place!)
