This is more historical review than anything else.
The whole TELECOM world is based on 64kbps DS0 Timeslots. The reason for this is the niquist theorem:
You need double the highest frequency to be able to reconstruct an analog waveform.
Taking voice ranges between 300Hz and 4Khz, you will need 8000 samples per seconds. If you take 8bit samples, you endup with your 64kbps. That's how the DS0 was born.
Initial SS7 system were composed of 64Khz DS0 that would aggregate to T1, enough T1 would agregate to a T3
672 DS0 lines (28 T1 lines).
You could buy multiple DS0 and allocate them as pleased. You might even not buy a full T1 at all, but X many channels. This is still seen today in fractional PRI, where you buy 10 Channels for example (rare since broadband has become so mainstream)
It's just the nature of how everything evolved, and more channels were been allocated. Eventually you could buy unchanneled T3 for a full stream of 44mbps transport (whoa bujaja). Then Frame relay, ATM and other came along to obsolete ISDN for data (still mainstream for voice) and what about CWDM and DWDM, ISDN had no competitor.
Best book on the matter is http://www.amazon.com/Telecommunications-Technologies-Reference-Brad-Dunsmore/dp/1587057972/ref=sr_1_2?ie=UTF8&qid=1382836993&sr=8-2&keywords=telecom+reference+cisco
