None of the original 7 and eventual 11 ordinances mentioned slavery as a cause of their decision to leave the Union ."
I'm sure you intended to say here: every 1st round secession document which gave reasons listed slavery most prominently.
Not every secession document gave reasons, so not every document listed slavery.
Here is a summary of six 1st Round "Reasons for Secession" documents
Note: the first four are official "Reasons for Secession" by South Carolina, Mississippi, Georgia and Texas, the only 1st Round states which issued reasons.
1st Round states of Florida, Louisiana & Alabama did not produce "Reasons for Secession" documents.
I'm including Robert Rhett because he wrote at the same time as South Carolina's secession to encourage other slave-states to secede.
CSA Vice President Alexander Stephens's words are the notorious "Cornerstone Speech".:
| Reasons for Secession | S. Carolina | Mississippi | Georgia | Texas | Rbt. Rhett | A. Stephens | AVERAGE OF 6 |
|---|---|---|---|---|---|---|---|
| Historical context | 41% | 20% | 23% | 21% | 20% | 20% | 24% |
| Slavery | 20% | 73% | 56% | 54% | 35% | 50% | 48% |
| States' Rights | 37% | 3% | 4% | 15% | 15% | 10% | 14% |
| Lincoln's election | 2% | 4% | 4% | 4% | 5% | 0% | 3% |
| Economic issues** | 0 | 0 | 15% | 0% | 25% | 20% | 10% |
| Military protection | 0 | 0 | 0 | 6% | 0% | 0% | 1% |
** Economic issues includes tariffs, "fishing smacks" and alleged favortism to Northerners in Federal spending.
Bottom line: across six documents, slavery accounted for 48% of the words explaining why they seceded.
No other issue came even close.
It's also important to notice that eight Upper South and Border States refused to secede so long as slavery was the only real reason.
Then after Fort Sumter four of those states declared secession based on Virginia's claim of "injury or oppression", not slavery.