I think the reason why the US war on drug is failing is deeper than just the incompetent and costly government policies.
I think the reason behind it is because the moral fiber of the American society has already eroded to such relevant level.
The significance of practical and traditional family values and social responsibilities are diminishing in favor of satisfying the personal needs for immediate gratification and escape from stress (which recreational and mind alternating drugs can efficiently provide to those who are addicted to it).
Its like a war where the people had already lost the will to fight and therefore there is no more point in continuing it.