I don't often disagree with Fred on issues like ISDN, but I'm going to advance a different theory: I had a chance to test an ISDN line at my home near Boston, back around 1994 or so, and I was /very/ surprised to find that getting a 64Kbps connection on either of the "Bearer" channels was very difficult.
It turned out that the only solution was to redial several times, and sooner or later I'd get a 64Kbps connection. After 15 or 20 minutes of dialing and redialing, I might end up with two 64Kbps "Bearer" connections to The Well, an ISP which served ISDN customers, and I could bind them together to obtain a 128 Kbps Internet connection.
When I investigated, I quickly found out that almost all of the T-Carrier systems connecting the central office to its Tandems were not equipped for "8 bit clean" connections. In other words, the connections from the CO to Tandem offices were designed for the original T-Carrier "robbed bit" signalling paradigm, and were not capable of delivering 64Kbps data connections.
I think Verizon - and probably the other Baby Bells - wanted to avoid the expense of retraining a unionized workforce to make use of the
8-bit-clean fiber-optic channels just being introduced at the time. The company would have had to retrain not only the "CO" technicians, but also the provisioning specialists responsible for specifying the number and type of trunks for each CO to use for each service. Even though ISDN data calls were billed per-minute, the accountants most likely projected more cost than revenue.Bill