Earlier quoted context omitted.
Why utilize scheme? If emacs has thaught us anything, it is that when you give people a fully featured language to fool around with, things you could never imagine can start happening. Why not scheme? It's liberally licensed and already has mind-share. It's a simple, no fuzz language with little obfuscating magic and great expressive power. It allows you to build your solutions as bare-bones as you like or go on full…
Might as well use python because of greater?? adoption and ease of use?
On the other hand, a LISP does not need a distinction between a declarative format and the computation language itself - it does both in a consistent syntax. (Homoiconicity is the term). It makes it much simpler to mix your declarations and computations, alleviating much of the need for a rigid specification on package definitions.
If you look at places where the former system is used, you'll find they usually age pretty quickly - or have many version increments because the original design omitted features people wanted in their package definitions. In effect, this leads to standards proliferation, because people invent new systems when the existing ones don't do what they need. Rather than trying to second guess the features people might need, it can be easier to just give them the ability to add them when they want.
A recent example of the former system is this project from GNOME: https://live.gnome.org/BuilDj. If you look at what it's trying to do, it really just wants to reinvent makefiles in YAML. A clear example of NIH, which will fail because as soon as someone needs to do something which their YAML schema doesn't support, it's a useless tool.