Because Lincoln applied the test of whether the United States courts and postal service were continuing to operate.
That was his pretext. He couldn't get a Southern governor to declare his State in insurrection, although he specifically invited at least one governor, Sam Houston, so to declare, by a secret letter. So Lincoln insurrected them himself by applying his little test. Which would have worked on any country in the world that was not, or was no longer, part of the American Union.
"Whereas, for the reasons assigned in my proclamation of the 19th instant, a blockade of the ports of the States of South Carolina, Georgia, Florida, Alabama, Louisiana, Mississippi, and Texas, was ordered to be established; and, whereas, since that date public property of the United States has been seized, the collection of the revenue obstructed, and duly commissioned officers of the United States, while engaged in executing the orders of their superiors, have been arrested and held in custody as prisoners, or have been impeded in the discharge of their official duties without due legal process by persons claiming to act under authority of the States of Virginia and North Carolina, an efficient blockade of the ports of those States will therefore also be established."
Sounds like a bit more than that to me.
Bump. Lincoln the usurper.