A number of states, among them California, recognized this and decriminalized possession of small amounts. Reagan's ascendancy to the top job in 1980 heralded a national shift to the right, and legislators responded with acts carrying harsher and harsher penalties for drug offenses. Simply put, a majority of Americans find marijuana morally offensive, although, if the studies are right, a third of them had to try it a few times before they could be sure. Humans have always used drugs, natural or otherwise, and it is paternalistic to tell us which ones are okay to use and which ones are not. A more nuanced offshoot of this school of thought suggests that the ultimate answer is to allow people to grow their own and use it themselves or give it away, but not to sell it.
Intelligent people can still disagree as to the health risks of chronic marijuana use.
Pro-marijuana folk blame the government for this, saying the government only gives lip service to the need for further study of pot, as it will not freely release it to scientists to study.