A Vox writer comes to the dawning realization that the US government isn't capable of handling the legalization of drugs. It's a pretty good piece -- I don't raise it to mock it, but to praise the willingness to rethink a long-held position based on evidence you would likely prefer to ignore.
The basic idea is that he has come to realize that, while legalization might work elsewhere, America's particular government is incapable of regulating drugs effectively. Legalization will thus predictably bring a vast increase in drug use and the trauma associated with it. It is not that it is impossible to legalize drugs and regulate them wisely; other countries may be able to do it. Our system, however, is incapable of it.
I sometimes use a similar argument against single-payer in America. It may be that other governments can do that well, but if we had single-payer, you already know what it would look like. It would look like the VA.