Upvote:10
The Swabian princes were "overthrown" in the Revolution of 1848 and forced by democratic forces to accept constitutional monarchies. When they couldn't get along with the democrats, they turned to the Prussians for military help to restore "order." Prussia was by far the most militaristic power in Germany and it had interests in protecting their Rhine territories from "revolution" even though the center of its power (Berlin) was at the opposite end of modern Germany from Swabia. And the fact that they were related to the Prussian rulers was a factor in the speed and power of the Prussian response.
At this point, the princes chose the "lesser of two evils" which was to hand over their principalities to Prussia (and keep it in the Hohenzollern family), or lose control altogether. The princes also made cash sales of their principalities that enabled them to maintain their lifestyles and a semblance of their former dignity.