They weren't. One of Lincoln's first efforts to "preserve the Union" was to call for the passage of the Corwin amendment which would have made slavery permanent.
This amendment passed both houses of Congress and was sent to the states for ratification. Five Northern states did vote to ratify this permanent slavery amendment. Lincoln personally wrote to the governors of the seceded states telling them that the Corwin Amendment was in the process of being ratified.
Again, this amendment made slavery permanent.
Here is more information, including the actual text of the amendment.
Also, whether conscripts or volunteers, they fought bravely and the outcome was, after a tragically high number of dead and wounded, union victory.
When you have four times the population and you even supplement that with grabbing Irish of the boat and forcing them into the army, it's pretty hard to lose.
Somebody should explain that to the Ukrainians and let them know they don't have a chance against the Russians. Except for the BS about the Irish of course.