Great question with a couple possible answers.

First off, while capitalists have class consciousness when it comes to keeping workers down, they are not a monolith. In the past (and still today) there are different factions within the capitalist class. Different factions compete with each other for investment and control of capital. Northern capitalists wanted more investment in their property, which was mostly factories. More money meant more machines, which could make individual workers more productive. They produced finished goods and needed free people to sell them too.

Southern capitalists resisted abolition because their capital was already heavily invested in slaves. When northerners invested capital, they bought more machinery or raw goods. Slavers just bought more slaves to work the land more intensely. Abolition would immediately make 99% of their investments worthless, even if they could eventually recoup the losses. It is the same short sided thinking that modern capitalists have, they prefer money now to money in the future. They were comfortable with the situation as it was, having the "free" north to sell their raw goods to at ridiculous profits.

But there's a cultural element to it. Southern life revolved around white supremacy for as long as there was an American south. Their world view revolved around the idea that black people were naturally subhuman. Anything less than that shattered their perception of self. That's why even poor southern whites fought to preserve slavery, it was the one thing they had. If they were poor and white, at least they weren't black and a slave. There's propaganda from the South that literally says as much.

There's an economic element to poor whites as well. If all the slaves were free, then poor white people had a huge new pool of workers to compete with for jobs. And for the slave owners, those free people might move north to find better jobs.

For your second question, the northern bourgeoisie largely did not want to go to war over slavery. Sure there were many genuine abolitionists, but most were comfortable with the relationship with the south. The standard Republican view was that eventually slavery would wither out because it was outdated. In the meantime Southern plantations provided raw cotton for cheap and the North could manufacture it and sell it. What the northern bourgeoisie could not live with, however, was this relationship changing. Lincoln was very clear in the early years of the Civil War, this was not a war over slavery it was a war over national unity. An independent South meant that Northern capitalists would have to pay tariffs and generally higher prices for raw material. Lincoln was clear, if the South had surrendered any time before 1863 he would have let slavery continue in the South. In fact the Emancipation Proclamation very clearly said that slavery was still legal in states that had not seceded from the Union. Northern capital would not profit without cheap southern cotton.

It was only after several years of warfare that it was clear to Northern bourgeoisie that the South would not surrender. At that point, the only way to stop the CSA was destroy their economic base. General Sherman's march to the sea liberated slaves and redistributed lands, it also tore up railroads and other infrastructure. Compromise was off the table and the North had to impose free wage capitalism onto the South at gunpoint (this also provided great investment opportunities for Northern capitalists.)

The North and the South saw the writing on the wall, free labor capitalism would eventually overtake slave plantation capitalism. The south were too high on their own white supremacist propaganda (see Social Darwinism) to see that peacefully transitioning into free labor would save them in the long run. So they tried to literally fight it. Enshrining slavery was the key feature of the CSA. Capitalists never willingly give up what they had and slavery was literally all they had.