weed is illegal because back in the day the drug companies were getting people addicted and loaded and pushing thier shit to make the allmighty buck and not caring about the consequences, just like today, and the religious right was going nuts because people were dancing and getting high and listening to jazz music and mixing races and having sex, and doing all the things that they shouldn't be doing, So the two teamed up to pass the buck.
At the time they started classifying and eventually made all drugs illegal, including alcohol. But unlike the other drugs, the booze industry had been a thriving business since forever and it didn't have the same social stigma and demonization that the other drugs got and continued to get up till today that led to them remaining illegal and the alchohol ban was lifted.