To answer this question, I believe some background on Venetia in Napoleonic context is necessary:

On October 27, 1797, the Treaty of Campo Formio was signed, formally ending the War of the First Coalition. In the treaty, the former territories of Venice were partitioned between Austria and the French Republic. The Venetian possessions in the Aegean and their Albanian territories were ceded to France, while Austria received the rest of the Venetian territory. (This was most likely compensation for losing control of the Low Countries in that same treaty.) Austria would go on to participate in the Wars of the Second and Third Coalitions, the latter of which resulted in the Treaty of Pressburg (French). Article IV of this treaty dictated that Austrian Venetia was to be ceded to Napoleon's client Kingdom of Italy.

From the signing of the Treaty of Pressburg to the Battle of the Mincio River in 1814, Venetia was solidly under the Franco-Italian boot. Thus, when the Austrians reclaimed Venetia following Mincio, they saw it as the reclamation of lost territory, not as an effort to restore the Venetian Republic. Seeing as Austria was one of the chief members of the Congress of Vienna, any proposal to give Venetia independence was doomed to fail. Furthermore, there was simply no reason to; the Coalition was not going to oppose Austrian strategic goals for Northern Italy in favor of reviving an ancient and anachronistic Venetian state.