A variety of reasons. The ongoing debate about federal power had tipped to the side of the 'Federalists,' for one thing. But also, and perhaps more importantly, the practical experience of the nation during the War for Independence had demonstrated that, without a stronger federal government, it would risk failing in its 'grand experiment' of republican self-governance.
Mining of fossil fuels and important minerals
This is known as the Anaconda Plan.
This military strategy, developed by Union General Winfield Scott, was meant to starve the Confederacy out of their resources. Essentially, Scott felt that if the Confederacy could not trade for goods with other countries they would not be able to survive. This is due to the Confederacy's reliance on cotton as their main cash crop.
This strategy helps to explain why the US wanted to blockade trade with other countries as well as why the Union wanted to control the Mississippi River, as they knew control of this waterway could negatively impact the Confederacy's ability to transport/obtain resources.
