Why did the nothern states not wish to allow missouri into the union as a slave state?

Answer 1

Because there are 2 Senators from each state, and if Missouri came in as a slave state, that would have given the pro-slavery states two more votes upsetting the then balance of power in the Senate where there were an equal number of slave and free state.

Chow!!

Answer 2

because that would upset the balance, and there would be more slave states than free stes, so anytime the staes voted on something it would always favor the slave states, so they comprimised