Maintaining the union is important. Without solidifying that every state is in it together, the federal government loses it's power, then states start to push the limits, not paying taxes here, disobeying federal law there.

If we wanted to let states secede, we would have formed a Confederacy, or not a country at all. As it stands now, the US government will always fight for a United States, as to not do so puts everything in jeopardy. Maybe you don't think that's the best action overall, but until the US government is reformed under a heavily modified constitution that probably won't happen.