Why were the cultures of the north and south so different despite the Union victory in the Civil War which united the country?

Because the culture that has been exist for hundred of years cannot be easily replaced with a single loss.

Explanation:

The whole southern society was build on slavery. They rely on slaves as their main economic labors in order to sustain their agricultural businesses.

Even after the union defeated the south, they southern citizens still kept a resentment in their heart toward the northern people. They refuse to assimilate with the black citizens which they regarded as beneath them. IN the end, the north failed to transform the southern culture. Even until know, both the culture in northern and southern states still very different.