Dr. Sussman's answer to "why did MIT siwtch from Scheme to Python in its cirriculum", in a nutshell is basically that MIT was orginally interested in teaching how the computer works from the high-level programming API down to the circuit level.
But sometime in the the world changed and he noticed his students were spending most of their time reading manual pages. Computers had become so much more complex that there was no practical benefit to learning computers at a lower level, everything was done at a higher level of abstraction. Even computer chips were so huge and complex that not a single person knew how they worked, the best you could hope for was individuals specializing in parts of the circuitry. Computers engineering became more of a "science" in that you learned its nature through experimentation, rather than at analyzing it down to its fundamental components.
So they decided to teach programming at a higher level, and eventually after 10 years they decided to use Python so that they could introduce people to the high-level scientific concepts they would need to become expert computer scientists.