56K really was the maximum for telephone modems. This is because the telephone network, internally, transmitted 8 bit samples at 8K samples/sec - 64Kbps - except the occasional low bit got stolen for other signaling, so the net data rate was a bit less (and the result was inaudible to a telephone customer). Instead of trying to synchronize the modem to the stolen bits, it was simpler to just use the 7 that were reliable - thus 56Kbps. https://en.wikipedia.org/wiki/Robbed-bit_signaling
9600bps as the defacto terminal speed was older than 9600bps modems (those were the cat's pajamas in the mid-late 80s or so). It was simply "fast enough" for a terminal user and "slow enough" for the terminals of the day. I had one (a Volker-Craig 4404 if I remember correctly) that could not keep up at 9600bps and had to be used at something less, maybe 4800bps.