Any fool can pull a trigger...
As the madmen play on words
And make us all dance to their song
To the tune of starving millions
To make a better kind of gun...
It's to do with the quality of the connection. The signal still travels through the line as an audio signal, not a digital one, and the sound at the other end is reconverted into data. 56K is the maximum current speed that data can travel through a normal telephone line before the signal degrades too much for the other computer to decode it.