Hardware was expensive. Even the software was expensive. Networks are cheap. Today, the only sensible way to charge for network usage is based on the size of the pipe. That's one price, per month, all month.
Faster lines cost more.
Not only does this kind of charging closely mirror the actual costs incurred in running the network, it makes sense to the consumer of the service. That's a rarity in today's business world. In the Internet, reality meets economics and they both come to the same conclusion. First you'd have to completely redesign the protocols, re- engineer the hardware, and retrofit the culture of the Internet. When it comes to bits, networks waste far more than they use.