To avoid causing drug induced psychosis in persons so prone, but who are not interested in drugs. Mentally ill people seek self-medication. They also seek others to medicate with - who wants to "toke" alone? - and that's why there is a drug problem. Drug addiction is a socially infectious disease. Those persons who had never tried drugs, and in the course of their normal life never would have tried marijuana, would be put at risk with premature legalization.
IOW, you're gonna be an ol' man when you will be able to go to the "quickie-mart" and buy a pack of "Beasters".
So the only people who seek to share drugs are mentally ill? I'd be fascinated to see the proof for this startling claim.