I've been reading the Chernow biography of Rockefeller, and this simply isn't true. We've almost never enforced "anti-trust law", and it's basically never been particularly effective.
The Sherman Act was widely considered a failure (even after passage in 1890). It did little/nothing to affect the fate of Standard Oil, which actually grew for a decade after passage, to over 90% control of the market by 1904. This is despite the state of Ohio engaging in a much more successful legal attack, based on technicalities of the trust charter, having nothing to do with the federal law.
The thing that actually brought down Standard Oil was...competition. By the time the company was actually broken up in under the Sherman Act in 1911, it had declined to ~60% market share. The overall story is essentially the same as today: the law ends up being used to punish declining companies for prior bad behavior.
Laws mostly don't work by actually filing cases. They mostly work by deterring malicious activity because people don't want a case brought against them. The cases are only for the ones who fail to be deterred, which is pretty uncommon for large corporations because they can afford lawyers to tell them what not to do.
The problem comes when you erode what was meant to be a strong antitrust law through decades of narrowing interpretations and then it's not deterring them anymore.
> the law ends up being used to punish declining companies for prior bad behavior.
The solution to this one is to take the politicians out of it and allow customers to file antitrust class actions.
Chicago school assholes backed by economic-right postwar "think tanks" managed to radically shift our standard for "harm" in anti trust enforcement, back in the '70s, though a combo of lobbying and positioning their folks in the right places. The result was that it became nearly impossible to take anti-trust action. That's why it seems like we suddenly stopped enforcing it—we did. This was followed shortly by a shift way to the right by Democrats on economic issues (among other things—listen to Clinton in the '90s talk about crime or schools or lots of other topics some time, he sounds exactly like a Republican) after Reagan's landslide, which put any hope of reversing that bad course on hold indefinitely.
The first notable push-back on that state of affairs was Lina Khan under Biden, who ever so slightly course-corrected us back toward healthy market competition in the tiniest of ways, and every business bro reacted like she was committing mass murder. Meanwhile, we need that times ten for at least a decade just to get us back to something resembling functioning markets. At this point I doubt we'll see a shift back toward reasonable market regulation... ever. Plutocrat capture of the levers of power is so complete that the only way I expect the US to see serious anti-trust action ever again is as a tool of corruption.
We also used to enforce antitrust law.