There are a few reasons I use Bash. It works very well as a "bootstrap language". Bash is good at setting up an environment and running complicated commands. Problems with Bash come up when you need to do anything slightly complicated, but for simple tasks it works very well.
Bash is a de-facto standard. It is available by default on all the nix operating systems I care about. My goal is to develop a stable, repeatable, and testable computing environment, and for this, Bash works. It can be relied upon as a platform to launch other software.
Bash is standard among hackers. Shell examples are given in Bash script. I can expect another programmer to be able to reasonably comprehend and modify a Bash script. In this way, Bash transcends cultural differences among programmers. I can use Bash skills on any type of project. Knowing Bash, then, is also extremely valuable.
I would love to use something else. Awk seems like it would be a very good choice, but I don't know it very well yet.
Ruby could be a good choice here, but I don't believe it currently is. Before I could ever rely on Ruby as a bootstrap language, I need to believe a Ruby script on one system will work reliably on another without having to dictate too much about either system's environment. I need to see binaries available that can be be installed anywhere. These binaries should be able to be placed anywhere on the system and still have everything work. Gems installed on one Ruby should not interfere with gems on any other. Environment variables from one should not interfere with environment variables from another.
Bash works well. I wish there were a better choice for standard systems scripting, but I don't think there is.