TIMEZONE defaults

Just curious; why are the default recognized <TIMEZONES>
not more comprehensive? 

Are timezone acronyms not standardized?
Do RFC's only recommend knowing a few timeszones?
Or is this a potential area for improvemnt?

Jeff

PS I didn't find an RFC or ISO standard during five miniutes of poking
around, but did find some informal timezone listings at:

http://sonne.net/Vicious/time.html
http://news.adventist.org/sun/