The debate over slavery resurfaced after the Missouri Compromise because the US wanted to make more territories into US states, specifically California.
The problem with making California a state was the fact that it was divided into two parts thanks to the 36'30 established by the Missouri Compromise. The Missouri Compromise established that any territory above this line would be a free state and anything below this line would be a slave state. California would technically be both if they followed this rule.
Instead the US Congress gets rid of the Missouri Compromise and replace it with the Compromise of 1850.
