Many reasons:
Jews made up a very small portion of the population (around 2%, if I remember correctly), and as a result, didn't have much political representation.
The Holocaust and the laws levied against Jews weren't passed all at once-- when the first few laws were passed, no one protested because they seemed harmless, and when the laws started getting restrictive it was too late.
Because Jews were such a minority, no one dared to speak up for fear of backlash/retaliation.
Those who thought something was wrong tried to spread the news, but many others didn't pay attention to them.
These are just a few reasons... one last thing to consider is that looking back, the Holocaust seemed inevitable, but it's a lot harder to tell if you were living DURING the time it was taking place.
