Microphones designed for human conversation in the 1950s and '60s were unable to react faster than 300 times a second, thus limiting reliable signal switching to 300 a second. Since this is a worst-case, this is what we get. The worst-case for acoustic couplers talking to carbon microphones is somewhere around 300 baud. 110 and 300 baud were chosen at the time primarily because both modems were intended to be used over copper wire and 'unconditioned' telephone lines, with at least one part of the connection going through an acoustic coupler.
