And that cheaper bit I think just came from reduced complexity. With things like ATM circuits and other similar highly reliable and predictable methods, they needed a lot of hand holding. You needed to provision an ATM circuit, you needed to make sure across the network that the path was there, capable, maintained, and configured, and you had visibility end to end

That was a selling point, because "hey we guarantee this circuit" but it was also very expensive and labor intensive

Where just dumping your bits into the internet and letting the network figure it out outsourced a lot of that complexity to every hop along the network you didn't own. But, because they care about their networks everyone would (in theory) make sure each hop was healthy, so you didn't need to hand hold your circuit or route completely end to end