What is a reason that the Missouri compromise did not have lasting effects

Being that this compromise decreed that whatever was north of the 36’60’ line was to be a free state and whatever was south of this line was to be slave state made this solution temporary. What would happen when the United States continued to expand West and reached the Pacific Ocean? There would be an imbalance of the number of free states and slaves states, with the free states outnumbering slave states. This flaw would be one of the most prominent causes to the Civil War four decades later.