Question

In the 1800s, why did the US government want to keep the number of free states and slave states equal?

Answer 1

Most likely to ensure that there weren't any new slave states, and to indirectly bring about slavery's end.

Answer 2

The reason there were even states was that there was even voting numbers in court (one vote for every state) If there wasn't an even amount of free states and slave states, then it would be very easy to one to overrule the other in court and making/passing laws. Basically it was there to keep peace.