The Bible isn't a legal document, this was regarding a law ...

This is very confusing, I didn't call it a legal document, I cited a law within it.

... for an ancient nation that doesn't exist anymore that was built on compromises with people who have hard hearts.

This is not a compelling rationalization, like at all. It doesn't land. The bible says lots of things are unacceptable, and enslaving people is not one of those things. It just isn't.

Christianity saw slavery for 1500 years and said "meh"

You have a misconception here that Christianity was popular for the first 1500 years.

No I don't. I mean Christianity didn't seem to have a problem with slavery.

The abolitionist movement was primarily driven by Christians.

... And primarily opposed by Christians. Yes Christianity piety was important to abolitionists, but slave owners received communion. Slave owners justified slavery's continuation by arguing that they were a saving the souls of the slaves, and they taught their slaves Christianity. Confederate general Stonewall Jackson was profoundly religious.

In the USA, slavery was essentially ended by the Great Awakening which was a time marked by mass conversion to Christianity among the peasantry.

It was ended (in America) by the American civil war. The civil war was caused by slavery, or more specifically southern fears of abolition due to shifting balance of demographics and economic power.