Presumably because most of the commands are just shortcuts for other shell commands. Probably the right tool for the job in this case.

Yeah, fair enough. Not everybody hates the shell as much as I do.

Someone would ask why not write it in sh, if it was written in Python, Ruby, Perl, or Tcl. I prefer stuff like this in sh.

I like to challenge myself to write little things like this as shell-scripts, too. But I am surprised they didn't at least choose use Bash instead of Bourne shell.