In the past most U.S. brewers used 6-row barley in a percentage of 50 to 85% for their beers, the rest being adjuncts like corn or rice. Nowadays more and more brewers use exclusively 2-row barley malt with no adjuncts added at all. This can not only be attributed to the change in beer styles and consumer taste but also to the progress in barley breeding.
Traditionally American beers were brewed with a large content of unmalted grain. In order to convert the starch from the grain into sugar, which is needed by the yeast for its living and for converting it into alcohol, one needs the enzymes of the malt. 6-row malt used to have a higher content of enzymes than 2-row malt, which made it the preferred choice for American brewers.
In the last few decades American beer culture changed rapidly with the rise of craft brewing. Craft brewers use mainly 2-row barley for their beers which is used around the world for high class beers. In many parts of the world 6-row barley is only used for livestock feed, not for beer.
But the difference is not only in the type of barley or malt but also in the quantity. Craft brewers brew their beers the traditional way with 100% malt and no unmalted grain at all. This does not make it necessary to use 6-row barley malt with its higher enzyme content.
Another reason for the greater use of 2-row barley in malt and beer is the progress in barley breeding. New varieties of two-row barleys, like Genesis, Synergy or Explorer, have caught up in enzymes to traditional six-row barley varieties like Tradition and still have a higher starch content, which gives a better yield to brewers.
The increased use of 2-row barley already gives the whole industry a major shift since craft brewers use 1.5 to 2 times more malt than industrial brewers, which calls for more malting capacities and farmers to switch the type of barley grown.