Tiggette
Because Politics. Research Prohibition and learn why banning alcohol never did and never will work.
It's one of those things that have been so deeply entrenched into the lives of the majority that a law holds no power.
Its not just politics. Its also social.
Socially speaking, alcohol has been accepted for a long time. Jesus almost certainly drank beer all the time because the water from the Nile was too much of a risk. Alot of our Founding Fathers were moonshiners back when it wasn't such a crime.
Weed however has been a sub-culture in the US pretty much since the 20's onward. Its not been really ever widely accepted, and people are always going to be more inclined to ban things they themselves do not use.
The main policy issue that most pro-weed people don't address too well is this notion of Pandora's Box. The fear is that, while weed is not as bad as alcohol, legalizing it would add yet another massly produced and abusable substance to the market. Anytime you increase access, you increase the likelihood of abuse. And what's more, the policing agencies for such substances already legalized are having a hard enough time managing that.
The reasons are multi-faceted, and go beyond the ol' hemp paper story we all for the most part know.