It appears that the declarations were widely published and available to anyone who could read. They were not hidden away in archives or library reference rooms as Mr. Goodguy opined. The original question was not about whether the war was or was not about slavery. The question was why, as you say, the documents were not "shoved into the faces of the South since 1866 by every Northern historian wanting to make a name for himself." We still haven't heard the answer.
You make a large amount of assumptions
Why would they need to? Anyone relatively contemporary to that time knew slavery was the motivation behind actions. Anyone knowledgeable of antebellum US history would already know that slavery was an increasingly sectional issue causing continued division. I think you made a comment about someone not being from Texas, I guess you are implying then you are. If so then if you really know Texas history you already know this, the massive issues with the annexation of Texas as a slave state and the power issues that would cause. This event was immensely significant in the building sectional slave division.
Additionally I'm assuming you are read on people like Sam Houston, though he even was against the war constantly railed against abolitionists and anti-slave sectional movements, blaming them for division (you can't have anti-slave groups without slavery).
All of these contemporary records are readily available and anyone within decades of the slavery era wouldn't need it shoved in their face, everyone knew already. People remembered and just started to not want to talk about it. Talking about contemporary views 150 years later seems enlightening but 20 years later it isn't nearly so surprising, it was common knowledge.
Additionally outside of the former slave holding States after Reconstruction many Whites wanted to move on so many avoided the slavery topic as well. They were tired of spending time and money on Blacks and "fixing" slavery.
Further it's odd to ask why for a negative, you seem to have a deeper point or agenda, you should just state that point and what backs up that point.