This is a consequence of computers + internet. 2 ways in particular:
1. Large organisations become unwieldy - computers help track and tame that complexity.
2. Geographically distributed organisations have trouble communicating and can be outcompeted in a region by a company focussing on that region. The internet helps reduce the friction in communication.
Once both of these started getting adopted in the 90s and 00s, big companies became more competitive relative to smaller ones.
And the increase of lobbies and their increasing efficacy in getting tailored legislation adopted. Arguably as politics has become more and more a business. Not just in the U. S
Monopolies and robber barons are hardly a new phenomenon. In fact, I've seen the term "robber baron" derided as an anachronism. I suppose oligarch or plutocrat are the modern terms. Nevertheless, economics 101 (literally I had to sit through this in freshman intro economics) will tell you about forces that drive businesses towards consolidation.
1. Large organisations become unwieldy - computers help track and tame that complexity.
2. Geographically distributed organisations have trouble communicating and can be outcompeted in a region by a company focussing on that region. The internet helps reduce the friction in communication.
Once both of these started getting adopted in the 90s and 00s, big companies became more competitive relative to smaller ones.