Instability. Ruby has not been the same language for very long. Migrating to 1.9 was a huge hassle for many firms. This may seem like a long time ago in tech years; but then there was Ruby 2.0; and shell scripts, meanwhile, have stayed the same the whole time.

A secondary reason is that Ruby has been very slow for much of its life, which means that for situations where you need to run a huge stack of scripts -- init systems, for example -- it would be punishing.

Ruby does have a terse and intuitive syntax that would make for a good system shell. Although it has some magic, it is less magical and confusing than shell itself. Ruby provides many basic data types that experience has proven are useful for shell scripting -- like arrays and dictionaries -- and they are integrated in a much cleaner and clearer way than they are integrated into widely used shells like Bash.

System tools that are written in Go may still make sense to write in Go, though. Go, it is true, does not have a nice terse syntax for short scripts and one liners; and it doesn't have a default execution model where everything is in main and so on; but that is because it is not a scripting language. Other languages used to write system tools and system services -- like C, C++, Java and Rust -- don't have those things either.