It's an user-experience problem: for an audiobook, Book of Why is quite dense in formula that are hard to visualize when spoke as words.
For example, consider what happens if we try to describe a causal diagram in words
"A points to B, A points to X, B points to Y, and X points to Y. Now, if we apply do(X) to the diagram, we see that we can Y is now no longer a child of..."
or even simple formulas in words:
"P of A given B times P of B is equal to P of B given A times P of A"
For most of us, this sort of deal is hard to "get" and would be much better served if we just looked at a visual diagram or saw the equation.
I personally had to repeat many sections over and over again with a notebook and pencil in hand to truly understand what was being read to me... but if I'm taking notes and creating visuals for myself, then I might as well have just gotten the paper variant of this book lol.
There are a lot of diagrams and a bit of math, which don't naturally translate to a purely auditory experience very well. Having read The Book of Why I can't imagine how it would translate to a reasonable audio-book. Frankly I'm surprised they even released an audio-book version at all.
