Because a previous regime had prepared the ground. The German Empire was the most highly militarized state in Europe and the press heavily censored. Right up to the end of WWI, the people believed they were winning it. That made it easy to believe the Army had not "really" lost it, but was betrayed by Jews and leftists on the home front.
This is an example of the same process. If you want to sell a lie, prepare the ground in advance.
This is an example of the same process. If you want to sell a lie, prepare the ground in advance.