why, God, why?
Perhaps due to the increasing popularity of JavaScript and the decreasing accessibility or usage of Scheme.
The fifth act of SICP is meta-linguistic abstraction, and among Javascript's merits it is not. SICP is no more a book about Scheme than Moby Dick is a whale tale.
I admire the effort, and understand the sentiment behind the project. I just think the authors are missing the big picture of SICP. In fairness, few of us are Ablesons or Sussmans.