The reasons I would cite: the Ruby development community leaves a lot to be desired from a sysadmin's perspective. We went through similar pains with Perl, but they're mostly a decade or two behind us. With Ruby, there are multiple concurrently maintained trees, it's hard to sort which one should be "primary" on a system, and you have to do a
lot of serious grokkage to understand that RVM isn't really intended for maintaining
system Ruby installs, but a local per-user
developer instance. Which, when what you've got Ruby for is managing chef, is a more than slightly whack.
http://www.lucas-nussbaum.net/blog/?p=617
http://www.lucas-nussbaum.net/blog/?p=681
http://news.ycombinator.com/item?id=2059964
Much of Perl's use is by sysadmins, which means it plays well in their space. By contrast, much of Ruby's use is by web developers, many of them on bastardized MacOS systems (they're born bastards, it's not the developers' fault ;-). Which means that Ruby's packaging is whack.
I don't hack much of either, though work around both. The nice thing about Perl from my perspective is:
1. It's included with the system. I run Debian/Ubuntu boxes if I can possibly help it, Perl is of of the tools used to write a whole slew of system scripts, and as a consequence, it's very well managed within the distro. Red Hat/CentOS aren't far behind.
2. CPAN plays nice. Under Debian, your CPAN bundles are installed to /usr/local/ Debian's just sorting out where to store Ruby gems.
Thanks to chromatic for mentioning perlbrew. I've encountered RVM in the past month or so and have been trying to wrap my head around it. Understanding RVM in terms of perlbrew helps a lot:
http://ithaca.arpinum.org/2010/06/13/rvm-and-perlbrew.html