This article is from the Esperanto FAQ, by Mike Urban email@example.com and Yves Bellefeuille firstname.lastname@example.org with numerous contributions by others.
(1) Esperanto is intended to be your *second* language, so it
remains relatively intact: people primarily create slang, idioms,
etc., in their native language.
(2) Esperanto is intended for cross-cultural use, therefore use of
too many colloquialisms, etc., jeopardizes your chances of being
understood (which is presumably your intention). This acts as a
stabilizing influence on the language.
Regional dialects appear when people communicate mostly with their
geographical neighbours and rarely with people from further away.
Dialects tend to disappear when long-range communication dominates
(as can be observed in many parts of the world after the
introduction of radio and television). There is also the not
insignificant observation that Esperanto has not formed any dialects
in its more than one hundred years of existence.