What they were fighting for was Federal funding.
They were concerns that the expansion of slavery into new territories would get slave holding States more votes in Congress, and more votes means more influence and more allocation of funds.
The North wanted to curtail the spread of slavery in said territory, not out of morality, but to keep things at least balanced between both regions in terms of government control.
Remember, they did benefit from slavery. Processing, manufacture, export - these required raw goods, much came from the South, and produced by the toil of slaves.
It's not an accident that moves to make slave illegal occurred AFTER the Civil War and during a time that the Union was trying to break the economic back of the South.