In response to Eileen's post, I'll try to help
explain based on what I know, if it's any
help.

First of all, remember that, after World
War I, Germany was in a state of disaster,
not just their economy but also their
national pride having been utterly
desecrated. Generally speaking, the
German people were happy to help
rebuild their glorious nation (which is the
way it was promoted.) Add to this the fact
that the Nazis main targets were
undesirables anyway, and that the Nazis
kept strict control over the flow of
information in the country, and it becomes
clearer how it was possible to keep the
knowledge of the camps secret. It was,
for the most part, a case of nobody really
paid much attention, because the only
ones being targeted were the political
and social undesirables, as well as
those who objected to the Nazi regime.
Also, most camps were build miles away
from any German towns or cities. Most
people really did believe that what the
Nazis were doing was the best thing, and
those who disagreed either kept quiet or
vanished. Besides, most people (and this
is true even today) believed that they, as
individuals, wouldn't have the capability to
do anything about the situation anyway.
Besides, the Nazis were helping to
rebuild Germany, and the only people
they were hurting were undesirables
anyway...or so the thinking went.

After the war, the German government
took a great deal of effort to sweep as
much information concerning the whole
thing under the proverbial carpet. After all,
would you really want it to get out that your
wonderful country had been committing
thousands of unspeakable atrocities for
the past several years? Also, for many
people, the memories of what happened
are still too painful to think about.

Whew. Hope that helps some (and
please feel free to comment.)