Languages become part of the standard installation by having very commonly used toos written in them.

I might be wrong, but I'm pretty sure ruby isn't part of a standard Debian installation, and neither is lua, for example. Ruby is very popular for web apps, but you don't need web apps to boot the computer, start a GUI and install packages, so it's not part of the standard installation.

But it's only an 'apt-get install ruby2.1' or so away, and so is rakudo (the current Perl 6 implementation). The debian package in stable is outdated, but work on newer versions is in progress. (Don't know about CentOS, sorry).