Prohibition was put into effect largely because of the efforts of Progressives who felt that drinking alcohol was detrimental to people and to the society as a whole. This attitude came about in part because of the industrialization of the country and in part because of the boom in immigration to the US.
Prohibition and temperance had been on the American social “radar screen” since the 1830s. Many Americans had always believed that alcohol was damaging to individuals who drank and to society. They had believed that individuals who drank lost control of themselves and were likely to harm themselves and their families.
This attitude increased in popularity as the US industrialized. Now, drinkers were a threat to the economy as well. In the industrialized America, workers needed to show up at work on time and to work consistently all day. Workers who were drunk or hung over might not live up to these standards. This made alcohol more of an issue for society as a whole than it once had been.
In addition, the influx of immigrants in the late 1800s and early 1900s increased the impetus for Prohibition. Many middle class Americans associated drinking with immigrants. They felt that alcohol helped to degrade the immigrants and make them more likely to commit crimes and to be bad workers. They also tended to think that immigrants were in some way undesirable. The drive for Prohibition, then, was in part a drive to control the immigrants and to make them behave in ways that were more palatable to middle class America.
For these reasons, Prohibition became politically popular and was established in 1920.