If you are in the US it is simply because not enough voting adults want it legal.

Most (even the non partakers) do think the current laws are a bit stiff but total legalization is too far. If you had backyards full lazy folks would try to steal like they do with any other "you got it I want it" commodity.
I've produced my own since '83 and had a glimmer when Ed proclaimed legal by 2004. Then he finally got hit,Emory got hit and I've still never seen a 14 footer basking in the sun rip itself out and go do something unspeakable to a child.

Logic is a strange animal. Sad but true.