The dissolution of was a consequence of Germany's defeat during the Second World War. There was a general consensus among the Allied leadership that Prussian history and social structures like the Junkers class or its General Staff inculcated militarism and an obedience to authority among the wider German public. In 1947, the Allied Control Commission issued a declaration that avowed:

The Prussian State which from early days has been a bearer of militarism and reaction in Germany has de facto ceased to exist.

Guided by the interests of preservation of peace and security of peoples and with the desire to assure further reconstruction of the political life of Germany on a democratic basis, the Control Council enacts [the disestablishment of Prussia as a legal entity]

This disbanding was done by fiat and without the consent of Germans.

The ideological justification was that Prussia embodied an aggressive militaristic tradition going back to the "Soldier King" Friedrich Wilhelm I which had allied itself with National Socialism and only its disestablishment would break the social structures to break this martial cycle of history.

On a more practical level, the westwards shift of Poland meant that the new Polish state had acquired a good portion of the historic core of old Prussia, including the much of the original Prussia. Initially, the Hohenzollern monarchs' title was "King in Prussia." The actual territory of Prussia, while an important component of the Hohenzollern patrimony, coexisted with other large territories like the Electorate of Brandenburg (which contains Berlin). The title of King was crucial for naming the rest of the territory Prussia, as it conferred upon the Hohenzollerns a status that their other rivals in Germany lacked.

Sources

Clark, Christopher M. Iron Kingdom: The Rise and Downfall of Prussia, 1600-1947. Cambridge, Mass: Belknap Press of Harvard University Press, 2006.

Dwyer, Philip G. Modern Prussian History 1830-1947. Harlow, England: Longman, 2001.