Answer:
To maintain the balance between slave states and free states.
Step-by-step explanation:
This happened back at the time when large portion of American citizens still think that slavery is normal.
At that time, United States were divided into two front, the Union (which consisted of free states that banned slavery) and the confederates (consisted of states which allow slavery) . both of these fronts tried to co-exist within United States.
The congress did not want to admit Missouri to the Union because a lot of its citizens were supporters of slavery. After Alabama became a state, it held the status as one of the largest slave holder in United states (with around 45% of population in Alabama were slaves). So, to tip the balance back in their favor, the Union wanted Missouri to be admitted as a free state.