I remember reading that the reason alcohol and cigarettes are legal while so many other (arguably less harmful) drugs are illegal is that cigarettes and alcohol have been a part of culture for so long so it would be a culture shock to ban these drugs.
The result of which would hurt the economy due to the loss of their sales and the increased spending needed to combat the illegal sales and consumption of these now illegal drugs. Even more politically, this would be an unpopular decision and could easily lead to a loss of votes for the party or candidate responsible in the next election.
Also @Holotor, you cant smoke cigarettes moderately for an extended period of time without serious detriment to your health. Alcohol really depends on the frequency but both are arguably worse for your health than marijuana.