I'm not exactly sure, but I wonder if beer drinking is simply down because other drinks have become more popular. For example, wine. Just like in Japan, wine in the US, a good wine has become a lot cheaper recently. You don't have to spend too much to get a pretty good tasting bottle of wine. In addition, maybe people are worried about their weight.
Beer has sort of a reputation for being a fattening drink.
Of course, all alcohol has a lot of calories, but people just think of beer as being a really fattening beverage. One more reason that I can think of is that there's probably been an increase in the past few years of students who simply don't drink at all. Over the past decade the negative impacts and the negative effects of drinking too much alcohol have been highly publicized, especially the binge drinking of college students. And so, a lot of people just for health reasons maybe don't wanna drink.