Because our government made a more successful attempt to demonize weed than liquor. In the prohibition era, there was a nadir of an extreme and racist brand of social conservatism which resulted in the banning of liquor and weed. Part of that demonizing of weed was giving it an exotic name that incured people's racist tendancies by the government using it's Spanish name, "Marijuana".
Of course, people were more loyal to their liquor than their weed because weed was less favored to the people of the prohibition era than liquor, and when the prohbition on liquor went away, the prohibition on weed didn't.
IMHO, I would never smoke the stuff again. It's become a feul to the fire of many of my illnesses. But as for other people using it, I think the government should stop looking in people's pipes and instead use those efforts to watch our boarders for the real criminals in our society.