Why did Prussia and Austria act without the broader German Confederation in 1864?

In 1864, Prussia and Austria acted as the two leading powers of the German Confederation to intervene in the duchies of Schleswig and Holstein following the Danish government's attempt to integrate Schleswig into the Danish state in 1863.12

Prussia and Austria took this action to uphold an international agreement—the London Protocol of 1852—which had guaranteed the autonomy of the duchies and stipulated that Denmark should not tie Schleswig more closely to itself than to Holstein.32

The decision for these two powers to act was driven by a desire to prevent the violation of this agreement. Furthermore, the Prussian leadership was motivated by a strategic interest in the region, and the Austrian emperor agreed to the joint intervention primarily because he was afraid to allow Prussia to act on its own. This collaboration led to the German-Danish War, which resulted in a swift victory and the subsequent joint possession of the duchies by Prussia and Austria.123