What are some of the reasons slavery was such an important issue to the North and the South?

Well, slavery was more important to the South because they had cotton to pick, hence the term: "cotton-picking". The North didn't even want to have slaves, but the South forced them to, but the North treated them more like people or indentured servants, so they were treated better and probably paid more. And if they weren't servants, they'd have their own shops.