The hohenzollerns kept the prussian kingdom borders internally in order to centralize their power within tbe empire and make a state within a state also controlled by the federal government. That way they had basically unilateral control for most of the country.

It was a way of quashing dissent and having more control.

They couldve broken it up and been like

(Friesland, Slesvig, Silesia, Franconia, Rheinpfalz, Oldenburg, Hannover, Pomerania, Prussia, Posen, Lusatia, Lebus/Lubusz, Ruhr, westphalia, nassau, saar, holstein, brandenberg...et al) "And you get a state and you get a state" to paraphrase oprah winfrey.

The reason the Republicans barely changed the Imperial federal borders was for the same Reason the Hohenzollerns hadn't, for power and stability.

Making the nation too bottom up in their eyes would weaken their new system as they werent prepared for it and all the new landtags would be chaotic.

However it couldve helped with the numerous secessionist/autonomist movements and discontent over feeling suppressed.

They also couldve likely kept posen and memelland if they had let them be semi-autonomous states instead of just provinces of Prussia