Virtualization, bursting and cost effectiveness all led to packet switching surpassing circuit switching in the WAN
Circuits as WAN connections battled successfully against packets for years because they guaranteed bandwidth, no matter what.
If you bought a T-1 circuit, the service provider nailed up 1.5Mbps from point A to point B, and that was your bandwidth come hell or high water.
Packets - first frame relay then IP - didn’t do that. They shared the network capacity, and service providers typically oversold access lines, knowing that if all customers tried to use all the bandwidth they bought at the same time the network would be swamped. Everybody’s traffic would suffer delays.
But frame relay service was relatively inexpensive compared with T-1s and it came in connections smaller than 1.5Mbps, usually in increments of 56Kbps. That meant sizing circuits to just handle demand, making business WANs more cost effective.
Plus frame relay services supported bursting - the instant delivery of more bandwidth than customers contracted for to handle moments of spikes in traffic. Because the networks were oversubscribed, sometimes the bursts were available, sometimes not. But over time, they were available enough of the time to make the service attractive.
Frame relay also offered virtual circuits - the logical carving up of access lines into lower bandwidth logical links tied to different sites. If a site needed connectivity to six others, it needed a single physical line subdivided into six virtual circuits. In the world of circuits, nothing was virtual. If one site in a corporate network needed to connect to six others, it needed six physical access lines.
As frame relay and later IP services matured, QoS was built into their standards so bandwidth could be guaranteed, just as it was in a circuit. That advance put packets over the top.