Some of it is lack of antitrust enforcement. Something that isn't mentioned much, though, is that business IT technology reached the point that planetary-scale companies work well. Until the 1980s or so, big companies had scaling problems. Big companies developed huge internal paperwork operations, huge headquarters staffs, too much middle management, and were choking on their own operating problems. There were inherent limits on bigness, beyond those imposed by the cost of transportation. The classic example was General Motors, which had a long history of management problems from sheer scale.

That's been fixed. Amazon, Google, WalMart, Alibaba, etc. have worldwide operations without major scaling problems. Those companies can adapt and change reasonably rapidly. The knowledge of how to do this is widespread. This is new. An inherent limit on bigness has been eliminated.