Why was it difficult to enforce prohibition?

The problem is similar to the difficulty in enforcing the ban on marijuana or stopping the sale of moonshine. Alcohol is easy to make and was legal in Canada and Mexico and Europe and so was easy to find and smuggle back into the US. There was a steady demand for it and the prohibition drove up the price and made it more lucrative to sell it. Many people did not feel any moral obligation to obey prohibition laws or report people who broke them. Often the local police liked to drink and looked the other way when they found liquor especially if they were given a share.