First - Slavery is bad, evil, no doubts about it!
Second - Although bad and evil it was LEGAL
Third - The economy of the North was based on large numbers of immigrants (thus negating the need for slaves or high wages for workers) working in a more highly technological area making textiles from cotton.
Fourth - The economy of the South was based on the production of Rice, Cotton, Tobacco and Sugar Cane. All of which at the time were very people intensive in that they needed planters and harvesters. Immigrants for the most part did not come south. Meaning that slavery was still acceptable because it was necessary to the economy. Machines to do this work did not exist at the time and were not even introduced until after the American civil war.
Fifth - The northern section of the Union wished to strike down terriffs against cotton and the other agricultural products of the southern section of the union, coming from India and other nations/colonies (which by the way were also being planted and harvested by slaves or near as much to slaves as it made no difference.

Now, knowing these five facts, can you just for a moment put yourselves into the shoes of a plantation owner that has been raised believing that slaves were property that you must now set them all free, losing your investment (you paid top dollar for them you know)with no recompense? With no guarantee that they will return to work for you at whatever wages you can afford? And at the same time the price of your commodity (the source of income and hence your ability to pay those wages) is going to drop like a stone because the northerners want to be able to buy cheap foreign imports?

I'm not defending the people who had slaves but I can sure see why they wouldn't want to just set them free.