The short answer is because of the same reasons the south was unhappy before the Civil war.

Access to road and port infrastructure. Lack of a Navy or merchant navy. Taxes on their goods and trade.

Border security would have been a nightmare, and it would have lead to escalating smuggling etc. as the Union would have attempted to impose tariffs and duties.

Both the North and the South would be required to arm and send troops to the border to guard them.

Add to the fact that slaves running to the Union would have immediately become free... and you have a volatile political situation.

We won't know what would have happened, but at the very least it would have set a precedent for splitting the union further every time that a state or group of states got upset.

The short answer is because of the same reasons the south was unhappy before the Civil war.

Access to rail and port infrastructure. Lack of a Navy or merchant navy. Taxes on their goods and trade.

Border security would have been a nightmare, and it would have lead to escalating smuggling etc. as the Union would have attempted to impose tariffs and duties.

Both the North and the South would be required to arm and send troops to the border to guard them.

Add to the fact that slaves running to the Union would have immediately become free... and you have a volatile political situation.

We won't know what would have happened, but at the very least it would have set a precedent for splitting the union further every time that a state or group of states got upset.