Did you know that pre-1980s, antitrust law was used _very_ liberally? There were many documented cases of the book being thrown at tiny companies looking to merge - the book that changed everyone's mind ( The Antitrust Paradox ) lists examples such as a merger between local groceries being denied as one of many great examples of antitrust law getting in the way of healthy competition instead of promoting it.
It seems that for most of the 10th century, antitrust law usage was on one side of the bar being used far too much, and as a reaction it's far too much on the other side - seldom used to the point where most industries are dominated by one or two giants.
It's just bad luck this transition seemed to have happened during a boom period of new industries and extreme growth.