Why did Prussia annex Hesse-Kassel in 1866?

Prussia annexed Hesse-Kassel in 1866 as a direct consequence of the Seven Weeks’ War between Prussia and Austria.1

During the conflict, the elector of Hesse-Kassel, Frederick William, chose to side with Austria against the wishes of the majority of his own legislative assembly (the Diet). Following the decisive Prussian victory at the Battle of Königgrätz, Prussia moved to consolidate its power and hegemony in Germany.12

Under the terms of the Treaty of Prague, which concluded the war, Prussia was permitted to annex several territories that had opposed it, including Hesse-Kassel, Hanover, Nassau, and Frankfurt am Main. Consequently, the elector was exiled, and in 1867, Hesse-Kassel was incorporated into the new Prussian province of Hesse-Nassau.21