Because it's deeply within the realm of personal responsibility. Look at the failure that is the war on drugs. You can't legislate behavior.
The war on drugs was never about legislating behavior. It was about criminalizing "undesirables" (blacks, hippies, etc.).
Regardless of the reason they did it, the end result was the same: failure to regulate behavior.
One can argue that the end result for the reason they did it is a resounding success. So their strategy was not focused in regulating behavior.
The war on drugs has been a resounding success at it's principle functions (none of which have anything to do with controlling drug use behavior). It helps maintain a permanent underclass which applies a downward pressure on wages via prison slave labor, allows the ruling class to more effectively criminalize dissent and propagandize against subversive groups, and provides an excuse to continue to militarize the police force.
The total US federal prison population working at jobs unrelated to prison maintenance numbers in the 10s of thousands, producing a value of $1Ms. A drop in the bucket, nationally. As an 'underclass' they are almost invisible. Don't know about the 'criminalize dissent' part but hey there's always the internet.