What was the motive of voters and politicians who supported segregation?

[Gracchus]

I think the racism just never left the south TBH. Since slavery was gone, they still wanted to keep them away from full rights. As for why their fear/racism, IDK. I guess they hadn't really learned to respect them, slavery or not.

Racism is alive and well in the North, thank you very much. The vast majority of Northerners didn't really want voting rights for blacks from the 1880s-1950s.

Agreed, a sense of superiority over blacks existed in both the North and the South at least up until the early 1920's, when the anti-lynching became an issue.  But in the South, it was likely just a general resentment towards Reconstruction and the North passed down over generations that led to blacks being the target.