Because an obscure little document called the Constitution prevented it.
Slavery in the United States lasted longer than in the Confederacy.
True. It ended in the Confederacy on January 1, 1862 and in the U.S. in December 1865 once the 13th Amendment was ratified.
When the CSA states left in 1861 the remaining USA states had more than enough votes to end slavery.
The states that had been interfering with that goal weren’t in Congress anymore.
And yet the ruling Republican Party waited until January 1865 to pass the proposed 13th Amendment and it wasn’t ratified until December.
What prevented them from doing this years earlier?
It prevented West Virginia from being formed out of the territory of Virginia, but they did it anyway. It prevented all the slave "property" in all the southern states to be seized without "due process", but they did it anyway.
They simply ignored the Constitution when it suited their interests, and demanded strict adherence to it when that suited their interests.
and in the U.S. in December 1865 once the 13th Amendment was ratified.
And another example of where they ignored constitutional law. In order to pass an amendment, you have to have *CONSENT* from 3/4ths of the states.
They created a fake "puppet" consent by using occupation armies to force states to vote for this amendment that they were *ORDERED* to vote for by Washington DC.
That is not how the constitutional amendment process works. Duress is false consent, and is therefore invalid.
They didn't care. They passed these amendments illegally anyways. Again, they ignored the constitution when they felt like it.