Because only the South actually did it. And their acts of secession, while illegal, weren't necessarily treasonous as the Constitution defines it.
Just curious here. For an act to be illegal it has to violate a law. Which law was violated by the Southern States' secession?
Secession wasn’t even illigal: per the 10th paraphrased “whatever (powers) not explicitly granted to the Federal government, shall be reserved to the states, and to the people of those states”.. Secession was not forbad, nor supremecy of the “Federal Government” over the states established: The Confederacy had a legal right to form and exist..
Their acts of secession were NOT illegal. If you can join a Union you can unjoin it. The constitution gives us the freedom to determine our own government and if that means leaving the Union then that is allowed also. The North violated the constitution, they were the treasoneous ones because they badgered the South until war was inevitable.
Trial for treason.
--The first treason case consequent upon the late rebellion is now being tried in the United States District Court of Tennessee before Judge Trigg. The case is the United States vs. John S. Gamble, who was an enrolling officer under the Confederate Government in Blount county, East Tennessee. This is the first treason trial since the memorable Aaron Burr case.
Gamble was acquitted.