Because alcohol contains only alcohol. The effects upon the brain are well known and documented. Marijuana, on the other hand, has hundreds of chemicals, each possibly interacting with the other(s) in ways not yet researched. For instance, a well-known drug test involves snipping off a bit of your hair. if you smoked pot even six months ago, traces of it remain detectible as part of the hair itself, vs. the kidneys elimination if all traces of alcohol from the body within 48 hours. Also, it is suspected that pot may alter the body's DNA, so long-term chronic effects are still unknown.
Why is marijuana illegal and alcohol still legal?