Why couldn't the Confederacy and the Union talk things out instead of going to war?

A really good question.

The failure to compromise is of course an answer and one that was offered by a well-known American historian and author, Shelby Foote. But I believe the answer is because of the law. In fact, legal issues, split into two parts would be the answer.

First, slavery. Allowed since the United States of America was created, slavery was practiced in the Southern states and some border states (such as Missouri) where African Americans known as blacks and worse names were not considered citizens. They could not vote or own property either. This legal status had been affirmed by the US Supreme Court in the famous Dred Scott decision. Though not considered a reason for the war at first, it later became a reason and for Abolitionists and blacks the cause for what President Lincoln said "a new birth of freedom".

Second, state rights. Basically, what was more important: the laws and rules passed by Congress and the President in Washington D.C. or those in the individual states such as Ohio and Alabama? And if a state disapproved of a law did it have to follow it? This legal issue was a big problem throughout the 1850s as new parts (regions or sections) wanted to join the Union as states. It was a reason for the South seceding from the United States in 1861.

Finally, you may find this interesting but shortly before the outbreak of war there was an attempt at talking in the form of a compromise known as the Crittenden Compromise and shortly after the South declared its independence was the Corwin Amendment. Both failed.

The two legal issues I spoke of were solved with the 13th, 14th, and 15th Amendments.

Hope this helps.