I know that the North was against slavery but what were the other reasons for this?
The Republican Party was founded as a sectional, Northern party opposed to the expansion of slavery in the territories. It continued to be the dominant party in the North until the Democrats started making in-roads during FDR's presidency. Between 1860 and 1930, and especially between 1860 and 1896, both parties were arguably more unified by ethnicity and sectionalism than ideology. A complex process of realignment began after 1930 as ideology came to the fore and the parties eventually sorted by ideology.
