Why....
The short answer is racism. At the turn of the 20th century, cannabis—as it was then commonly known in the United States—was a little-used drug among Americans. With the start of the Mexican Revolution in 1910, however, many Mexicans began moving to the United States, and they brought with them the tradition of smoking marihuana. Amid a growing fear of Mexican immigrants, hysterical claims about the drug began to circulate, such as allegations that it caused a “lust for blood.” In addition, the term cannabis was largely replaced by the Anglicized marijuana, which some speculated was done to promote the foreignness of the drug and thus stoke xenophobia. Around this time many states began passing laws to ban pot.