In the United States, many medications are much more expensive than in other countries (see for example How the U.S. Pays 3 Times More for Drugs). This is a serious problem for many patients, and becoming a political topic (Lower Drug Prices: New Proposals Carry Lots of Promises, New York Times article from February 2018).
Given these facts: Why don't the US just import the medications from where they are cheap? This would seem the obvious answer, particularly in a country where free trade is highly valued. It also seems to work nicely for many other goods, such as clothing, cars or electronics.
If I understand correctly, one obstacle is that it is generally illegal to import medication into the USA, because that medication lacks the required approval by US authorities, specifically the FDA (Information on Importation of Drugs prepared by the Division of Import Operations and Policy, FDA).
However, this still leads to the question of why this is not changed? Laws can be modified - is it protection of US pharmaceuticals, concern about quality, something else? It would seem a no-brainer to lift restrictions that increase the cost of medication.
Note that there is precedent: In the European Union, approval of a medication in one country is generally accepted in other countries, too (with some limitations) - see e.g. the page on Authorisation of medicinesfrom the European Medicines Agency.