Perhaps it's not fair to point to a 97-page article as "something specific," but "the thing that changed" was thoroughly explained in Amazon's Antitrust Paradox (2017) [1]. Specifically:
> In some ways, the story of Amazon's sustained and growing dominance is also the story of changes in our antitrust laws. Due to a change in legal thinking and practice in the 1970s and 1980s, antitrust law now assesses competition largely with an eye to the short-term interests of consumers, not producers or the health of the market as a whole; antitrust doctrine views low consumer prices, alone, to be evidence of sound competition.
> In some ways, the story of Amazon's sustained and growing dominance is also the story of changes in our antitrust laws. Due to a change in legal thinking and practice in the 1970s and 1980s, antitrust law now assesses competition largely with an eye to the short-term interests of consumers, not producers or the health of the market as a whole; antitrust doctrine views low consumer prices, alone, to be evidence of sound competition.
[1]: https://digitalcommons.law.yale.edu/cgi/viewcontent.cgi?arti...