Why was it imperative to the allies to end the monarchy in Germany? Why not just appoint a new monarch?

by mystical-me
iambamba

The Allies did not, in fact, end the monarchy in Germany. The monarchy was toppled by an internal revolution just as the German government was agreeing to an armistice. It started as a naval mutiny, but quickly spread, latched upon by leftists and the rest of the repressed political opposition. Seeing the writing on the wall, the military advised Kaiser Wilhelm to abdicate. There was no more hope for the dynasty and he quickly fled the country.

Both Germany and the Allies assumed a continuity of government despite the new republican regime, which is evidenced by the fact that the name of the German State, Deutsches Reich, remained unchanged essentially until 1945.

Wilson's Fourteen Points were concerned not with toppling monarchies, but with self-determination for the people of Europe who lived under foreign rule. This includes the Poles of the Posen region ruled over by Germany, but mainly referred to the myriad minorities under Austro-Hungarian rule in Central and Eastern Europe. Whether or not the Hohenzollern and Habsburg dynasties were maintained was not of immediate concern.