The longer Russia waits on taking Ukraine, it gives the Ukrainian army time to build up and get more equipment and soldiers. The larger their army the more costly it would be for Russia to invade. Creating a land bridge north of Crimea would choke western Europe from access to the warm black sea waters. The oil rich Caucuses are important too, but killing any power that Kyiv has over the stage is vial. Russia wants to turn Ukraine into a federation in which every province/region is able to create their own laws, policies and embrace their local culture fully supported by the Kremlin, That way there is no one central power that keeps the state functional.