Alcohol is bad for you. Although it's been drunk by human beings to either celebrate the good times or drown their sorrows for hundreds of years, everybody knows that drinking too much beer isn't good for your long-term health. Previous generations didn't get as many health warnings about the dangers of beer as millennials have been exposed to, and statistics indicate that they've been listening to the warnings. They don't drink as much as their parents.
When millennials do drink, they prefer spirits or wine to the humble beer, and as a result, brewers are counting the cost of this change in tastes. Between 2006 and 2016, wine and liquor took ten percent of the alcohol market share from beer, and that trend has been continuing at around 1% per year since. The fact that a pint of artisan 'craft' beer is often more expensive than an entire bottle of wine might have something to do with this.