There were many reasons. For starters, it involved getting rid of a large number of Native Americans, and while some supported this, others opposed it as they were peaceful. Another thing was that this meant the introduction of new states and it deepened the division based on whether the states should be slave states or non-slave states. Also they involved war with Mexico and some wanted this and some didn't.
Ima guess right now and say that its due to what they bought was cheap
That is was the key stone to their life and they needed to protect it at with everything they had to offer,
Well its basically when two states are arguing about something and it goes overboard
