Abelson is showing how a bunch of stream functions resemble a boxes and arrows diagram of communicating processes, but he has to start from the back, and the calls are all nested into a deep expression. The structure is not evident at first glance.

With transducers each box would be neatly on the same level, you just compose them in sequence, and then you feed that to the transducing function. It looks a lot like a shell pipeline, and not like nested calls.