Nanoc’s use of Ruby is not an appealing choice these days
Nanoc is written in Ruby and does not hide its Ruby roots:
Nanoc requires Ruby to be installed. It is also recommended to install Bundler (which does not come with Ruby by default) to manage optional dependencies.
Nanoc requires knowledge of Ruby to create anything but the most trivial of sites. The heart of a Nanoc site is arguably the Rules file, which is written in Ruby.
These two points combined makes Nanoc not an appealing choice for people who do not want to manage a Ruby installation (with dependencies such as Bundler), and/or do not want to learn and maintain their knowledge of Ruby as a programming language.
Nanoc was written in Ruby’s heyday, and Nanoc’s initial release followed a little after Rails’ 1.0 release. The attention that Rails and Ruby got from the web development community made Ruby a great choice at the time.
Replacing Ruby with anything else is a ton of work (not just the rewrite but also providing a smooth migration path). Languages like Go and Rust might have been a good choice at the beginning of the development of Nanoc — if they had existed.