simplest answer: the 1996 Telecom Deregulation Act.
Sold as a way to bring real competition after the breakup of ATT, what it did instead was: 1) coax Baby Boomer money into the stock market, causing the Telecom boom, with all the mis-investing that a boom entails, 2) build a lot of infrastructure with the new money, that the incumbent companies didn't have the stomach for, 3) free up the baby Bells from much regulation, but not enforce the "play fair" rules to allow upstarts to use the extant infrastructure, and 4) crash the boom, allowing the incumbents to pick up intrastructure for cents on the dollar. Instead of a rebirth following the crash, as is usual, in which the newcomers get to pick through the remains of the last boom, the remants mostly got gobbled up by the incumbents (even ATT, absorbed by SBC). So the landscape looks much like it did originally, except now the old incumbents control the Internet, instead of just the phone system. Innovation is largely coming on top of the Internet, instead of in providing it.