These modems are from 1999 or later, so not exactly ancient. Now I'm wondering if the limit to modem speeds in the 80s and 90s was encoding design or the cost of DSPs to implement more advanced encodings.
I think it's mostly due to the fact that modems used just the audio bandwidth of the telephone line, which is about 3 kHz (by design). This limits the maximum speed, earlier versions probably had problems with transceiver complexity.
