Why didn't private grumbling become open opposition?

Opposition was divided, communists & social democrats didn't trust each other.  There was no Organised Opposition.  Nazis had removed or taken over nearly all organisations, people couldn't join groups to resist them.

Gestapo could destroy peoples lives, the germans were afraid.

People didn't know what was going on.  Unpopular Nazi policies were either dropped or kept in secret.  Propaganda and censorship made sure extremes of Nazi Policy were kept secret.

People tolerated the Nazi policies for the stability and prosperity they believed was happening. Nazis achieved electoral success so most people thought the could do whatever they wanted. Those dissatisfied with the Nazis often had minor quibbles.