Quoting from this article at http://www.brewersassociation.org/
America's brewing landscape began to change by the late-1970s. The traditions and styles brought over by immigrants from all over the world were disappearing. Only light lager appeared on shelves and in bars and imported beer was not a significant player in the marketplace. Highly effective marketing campaigns had changed America's beer preference to light-adjunct lager. Low calorie light lager beers soon began driving and shaping the growth and nature of the American beer industry, even to present day.
By the end of the decade the beer industry had consolidated to only 44 brewing companies. Industry experts predicted that soon there would only be 5 brewing companies in the United States.
Now, craft brewers operate in 344 congressional districts and the majority of Americans live within 10 miles of a brewery.