Alcohol is legal because it has been used pretty much everywhere since time immemorial. Weed has been used also since ancient times, but only in very limited areas and not in the Western societies (that I know of, at least). Alcohol is even used by the church, which used to be the meter of right and wrong for a long time. Drugs were introduced later and were thus easier to ban as they didn't have such a widespread penetration in the population.

Things like this have the additional complication of already being banned by the law a long time ago and thus needing a majority in the parliament to be ignored by the law once again (or allowed and regulated, like it most likely would be). It could be a political suicide for any old politicians to start to advocate drugs. I reckon most voters still don't use drugs so they wouldn't care to have them legalized, thinking "since I don't smoke pot, nobody needs to".