-Lack of demand -Hardly any servers running Linux locally or nationally
http://netcraft.net used to have a report on operating system usage in web servers.
The demand also doesn't tend to be public. At the Health Sciences Centre, we use Linux for several IT functions. Hardly anyone knows, because as long as it works, no one cares. The difference is in the bottom line.
-No official training materials
There are several certifications out there, such as LPI, CompTIA Linux+, SAIR, and RHCE. They all have books outlining a course of study. Cramsession.com even has condensed study guides for most certifications.
-No need for Linux administrators
Maybe, maybe not. Remember that Linux skills are easily transferrable to other Unix variants. Someone well versed in Linux would have an easy time picking up, say Solaris. Linux is a good OS to learn on. Lately, in job postings I see on some mailing lists, though, Linux has been listed as a "nice to have".
Methinks your instructor either has his head in the sand, or does not wish to have to learn something new to teach you.
Sean