This should be so easy to explain that even an historian could understand it.
When the Constitution was written, southern states were given representation in Congress based, in part, on the number of slaves in their states- thus giving every Southern man more power in Congress than any Northern man. For about 50 years the power of the South in Congress was unassailable by the North.
During that time the South as a region became addicted to an economic power that terrified them. Slaves were valuable property that could escape or murder their owners in their beds. Southerners made it illegal to teach slaves to read, write, or even speak proper English, thus creating a large class of people unprepared for the tasks of self-government.
When immigration populated the Northern states and the balance of power in the Congress began to shift, the problem for the South became plain- slavery could only continue to exist if either more slave states were admitted than free, if the North could be persuaded to countenance slavery, or if the Southern states seceded.
How all of this played out can be recounted in plenteous detail, but the above, in a nutshell, is why slavery was the cause of the Civil War.