For the authors of this book, I imagine it was a combination of Scala having the features they wanted, plus it being relatively popular, easy to install, and having good tooling.

I've written a lot of Scala in the last 15 years or so, and I really like the language. It has features that most programmers are not familiar with, which can scare off some. My opinion is that if you understand the features it is a very elegant and simple language, particularly Scala 3.