A few things to mention here. First, the government actually paid a university to grow cannabis and they gave it to glaucoma patients for decades without any interesting results (in my opinion probably because the grantees were more interested in getting repeated funding from NIDA than to publish things NIDA didn't want to see). Second, there have been a number of clinical trials on marijuana for quite some time with a wide range of results (it's a tricky drug to study for a number of reasons). For a long time, NIDA made it hard to get funding for this- and also constrained the supply- probably with the goal of preventing studies that would find efficacy. Third, I think the FDA system is really poor at evaluating a wide range of drugs (of which cannabis is an example). They have a really limiting mechanism that doesn't work well in "compassionate use" where we already have a good understanding of the risks from widespread use. Fourth, the FDA wasn't created to do this- it was created mainly to reduce consumer fear of drugs in response to absolutely horrific side effects. The drug that led to this, thalidomide, is
approved by the FDA and used in the US as a highly effective medicine.
I think in retrospect, it took the FDA so long to get here for political reasons, not scientific or medical ones.