Why do so many (mostly Republicans) have such a hard time realizing the Civil War was about slavery?

They don't read. They repeat whatever the TV tells them.

The president of the Confederate states Jefferson Davis put in writing that the main reason was the "right to own people". Of course they probably think he was a liberal.

It was not just about slavery. Slavery in the South was one aspect, all be it the most important by many accounts, but not the only reason.

Amazing 2nd-grade-level grasp of U.S history you have there!

Becouse it was only one part of the entire picture. Why is it that the left so often picks and chooses one part of an issue and only focuses on it?

White Southerners don't want to admit that their ancestors fought for an evil cause. So they have to make up reasons for why the war was fought that have nothing to do with slavery.

David Blight has done a lot of work on mythology of the Civil war with his book "Race and Reunion". The thesis is basically that, after Reconstruction, Americans prioritized reunion. In the decades after the war they came up with a narrative which stressed the shared valor and glory of the combatants and downplayed, or even erased the radical political content of the war. For this to work they had to downplay slavery as a cause since it is clearly wrong.

it was the democrat slave owners versus the GOP states who opposed them.

It primarily was about preserving the Union, when the South seceded.

They prefer to defer from the truth.

And....they failed U.S. History.