Why the slaves in south is more important than North? (civil war)?

There was no slavery in the north at that time, and a number of influential northern folk wanted slavery eliminated throughout the country.

Early on the Southern economy was geared heavily around the production of cash crops for export. Initially, beginning oin the 17th century and continuing through the 18th, this was primarily tobacco (although there were some other crops such as indigo, rice, and in some limited areas, cotton). The best way to grow this was through gang agricultural labor. But the abundant land, and chronic labor shortage in America meant that it was relatively easy for free and indentured laborers to strike out on their own. In order to keep their work force more stable, Southerners turned towards the importation of enslaved Africans.

Prior to the Revolution, slavery was legal throughout British North America, but it wasn't as central to the economy in the North. In part this was due to immigration patterns being different. In the 17th century, New England, and to a lesser extent the rest of the North, saw more families immigrating, while the South saw more single men. This meant that the North was able to replicate a European style nuclear family much more easily and were quickly able to establish societies which expanded through reproduction, ie people having kids. For a variety of reasons, agriculture in the North was more focused around family farms than large plantations. Labor was mostly done by members of the family with hired hands helping to fill the gap. Slaves were a part of this mix too, but were more peripheral to the labor system in the North, whereas in the South they were central to it. Some of this had to do with the type of agriculture being practiced too. In the South, the crops they were growing had labor regimes which required fairly steady levels of work year round. In the North, the concentration on growing grains meant that there were bursts of labor in the planting and harvesting times but less labor in between. In those circumstances owning your laborers wouldn't necessarily make sense since you'd have to invest in them all year round but only get the benefit of their labor twice a year. Better to hire free laborers who you can let go in the off season.

For these reasons, slavery was always less important to the labor scheme in the North than in the South. The American Revolution brought a reassessment of the place of slavery in America. The Rebels had fought a war which was rhetorically dedicated to liberty and the idea that "all men are created equal". They weren't all blind to the hypocrisy of using that phrase while holding slaves. In the last two decades of the 18th century this led to a series of changes. In the South you saw a small trend towards voluntary manumission where some slaveholders, most famously George Washington, agreed, usually in their wills, to free their slaves. Thousands of slaves were freed this way, forming the nucleus of a free black population in the South. But Southerners, for the most part, still wanted to hold onto their slaves and there was no move to try and restrict, let alone end, slavery. The same wasn't true in the North. Since slavery was more marginal there, it was easier for its foes to attack the institution. This gradually led all of the Northern states to eventually abolish slavery. Sometimes, as in Massachusetts, this was done by court case (in the cases of Quock Walker and Elizabeth Freeman, the Massachusetts Supreme Judicial Court ruled that slavery was unconstitutional in the Commonwealth). More often, as in New York, it was done by act of the legislature (New York decreed that all people born after a certain date were freed and that all slaves would be freed by, IIRC, 1821).

Conversely, slavery became more deeply entrenched around the same time in the South. The primary agent of this was the invention of the cotton gin. There had long been some cotton grown in the South, but it was more of a niche product which could only be grown in certain areas. The type of cotton which could grow throughout the South was simply too difficult to clean in an economical fashion. The cotton gin changed this. It mechanized the cleaning of cotton, which allowed it to be grown economically across virtually the entire South. What resulted was a cotton boom where cotton production came to eclipse tobacco and other products as the staple crop off choice in the South. With iindustrialization in Europe (and a lesser extent in the North) and with growing globalization there was incredible demand for cotton to make textiles and the Southern economy boomed. Slaves, and the products they produced, became more valuable than ever and slavery became more entrenched in the South across the 19th century. In the 18th century it had been somewhat common for slaveholders to see slavery as a necessary evil: a cruel and dangerous system which they nevertheless felt they could not get rid of. In the 19th century, that view became taboo in the South as slavery was portrayed as an unalloyed good. Over time, Southern states made it virtually impossible for slaves to be freed, even by their owners, and the South became more and more hard line in their defense of the "peculiar institution".

The two major crops in the agrarian South were tobacco and King Cotton - both labor intensive.
In the more factory/business oriented North labor needs were provided by non-slave workers.