I know that the North was against slavery but what were the other reasons for this?
Well, firstly the entire North wasn't solid Republican. The inner cities, where many Irish and Italian immigrants lived, were pretty Democratic. Additionally, the "western" (for the time) states in the Midwest were swing states, with a lot of German and Scandinavian immigrants. The main Republican base was New England, which had a higher percentage of WASPs, a group that had a higher socioeconomic status than most in Nineteenth Century America, which (in the North) was tied to more Republican voting. Keep in mind that NY State could outvote NYC well into the Twentieth Century, MA could outvote Boston, etc. Northern corporations that obviously operated in a slavefree economy were also very united against the expansion of slavery (in an admittedly moderate hero way, as they didn't want to REALLY antagonize Southern planters), and they were pretty effective at convincing manufacturing workers and others in the North that it was in their interests to vote Republican.
The North was never ANYWHERE near as Republican as the South was Democratic; there were just a lot more Northerners.
Best description I've heard that effectively smashes the "Parties flipped" narrative pushed by the left
