Tariffs as a means of influencing standards involved in production processes are 1) difficult to define and also difficult to implement, 2) reduce ties and increase probability of any given issue leading towards heightened conflict or even war and 3) most importantly, they are liable to backfire by enabling a dictator to blame foreigners for hardships experienced locally.
1) It would be difficult to (accurately) tax products on the basis of the labour standards and other standards involved in their production. Even taxes that appear theoretically useful become politically infeasible when they are difficult to define with specificity, let alone actually apply.
2) Tariffs too easily lead to escalations, such as trade wars, which reduce ties between countries and thus make it easier to go to war. They absolutely remain in the toolkit, but should be seen as a last resort, generally not to be used (excluding broader revenue raising considerations, such as in countries where the state actively controls the main border points, but not much else) except in the most serious of situations.
3) Also, massive tariff hikes (or sanctions) make for scapegoats, and foreign governments will often be able to succeed in blaming Western countries for sanctions or tariffs as causing hardships, rather than having the desired effect of them improving standards. Foreign populations will more easily believe the moral dimensions — including our rational self interest that we can uphold standards more easily if they have higher standards — if this does not come in the form of a tax that allows dictators to blame colonists and multinationals for the hardships of everyday people.