From the 1960s to 1980s the brewing industry experienced a handful of factors that caused changes in the market control. One such innovation was the development of faster canning machines. From the 1950s to the 2000s, the canning speed capabilities grew in a logistic fashion where the steep section overlapped the 60s through the 80s. This increase in canning capacity allowed for an increase in production. This benefit, however, was only felt by the companies that were already making significant profit margins because these new machines were relatively expensive.
Not only was the machinery expensive, but also the time to install would cause production outputs to decrease. This is another reason why only large breweries could afford the new lines. During the install period, these breweries could switch capacity to other lines and still produce enough to make nice margins. If a smaller brewery attempted this, they could be stuck severely reducing production capacity until the new line was up and running.
This trend is seen in the spread of market shares. In the 1960s there were thirteen major beer producers controlling only 53.56% of the US market shares. However, by 1982 there were only eight major companies left standing which held 92% of the shares, of which, the top four held 75.84%. Granted this shift was not only because of the innovations in canning lines, but I am positive that it played a part.