While Germany became a relatively unified state in 1871, there was still a considerable amount of federalism. Except for a few small (compared to the overall area and population of Germany) city-states, all constituent states were monarchies, with different amounts of constitutionalism and parliamentarianism.
How come that within a few weeks in 1918, all these monarchs abdicated, and the monarchies were transformed into republics? Given the number of constituent states, one would think there would be at least a few whose population would be content enough with their monarchs to transform into democratic, parliamentarian monarchies. Were all monarchs, without exception, so unpopular that a shift to a republican order was seen as the best way forward in all German states? Or was there some leverage from the central power, powerful enough to enforce abdication in all constituent states?
Basically it reached a point where the allies (especially Wilson) only wanted to negotiate a peace agreement with a democratic country/ if Germany was to become a democracy. In the end Wilhelm 2 was too undecisisive and didnt retire fast enough in the eyes of the social democrats. So they just decided to retire himself more or less since the democratisation process had to be quick. This lead to the famous twice announced republic by Scheidemann and Liebknecht within merely 2 hours.