One of the problems with the modems circa 1960 was that AT&T felt they should operate over nearly any dialed-up connection between any two points. With the state of the telephone plant in the early 1960s this was a tall order. There is a paper (which I can't readily reference) where Bell Labs did a bunch of test calls all over the U.S. to assess the ability of their modems to communicate with acceptably low error rates. This would be a requirement for TWX, offering nationwide service, and for higher-speed modems. At higher speeds typically a corporation would make calls all over the U.S. to collect data from its local offices.
The makers of acoustic couplers and would-be makers of third-party modems realized that for the bulk of the market - computer time sharing terminals -- most connections would be local or nearly so and they could get by with a much lower-performance modem. Meanwhile the telephone plant was rapidly improving and the probability of getting an unusable connection was steadily going down. So the Bell modems were overdesigned and consequently overpriced for the kind of service that much of the market needed. --
jhhaynes at earthlink dot net