Because of how powerful is Perl? Because of the amount of things that
depend on Perl that currently exist and would be a waste of time to
rewrite? Because Perl might be the best tool for many cases? There are
many possible answers...
Not that coding in Lua, scsh or similar tools is bad. Not that the
regular shells are not up to the task in many cases. Simply that... Perl
seems to be such a powerful language that people would end up installing
it anyway - And if you have an optional package installed in 95% of the
machines, then you have something I would like calling 'base'.