I am not one of the authors, I can only speculate :)
I think the most important factor was the ease of understanding and implementation. The role of this course is to give you a working overall view of a large chunk of how computers work. There are many possible improvements to the CPU, but most of them would require a much more complicated design.
Similar considerations went on with the design of the Assembler, the VM, Jack and the OS. In the end, students need to be able to implement a working computer along with an OS and a software development stack in 2 semesters.
The designers' thoughts about the Hack Computer architecture are expressed in the Chapter 5 Perspective:
Following the general spirit of the book, the architecture of the Hack computer is rather minimal. Typical computer platforms have more registers, more data types, more powerful ALUs, and richer instruction sets. However, these differences are mainly quantitative. From a qualitative standpoint, Hack is quite similar to most digital computers, as they all follow the same conceptual paradigm: the von Neumann architecture.
Since the Hack Computer is specifically for educational purposes, goals and priorities would have been something like:
1) Implements general-purpose stored program computer,
2) Simple to understand how it functions,
3) Simple to implement in HDL.
I continue to be amazed at how well the authors achieved these goals.