Why does the Nile river flow from south to north?

The Nile flows north because north is downhill.

Rivers flow "downhill" to sea level (generally speaking; some end in other bodies of water or join another rver). They flow with gravity from a higher elevation to a lower.

The Nile rises on the western flank of the Great Rift Valley (many people consider Lake Victoria to be the source) and the Blue Nile joins the White Nile at Khartoum.

The land falls away towards the north, thus the river flows north until it reaches the Mediterranean Sea at the Nile Delta. Direction of flow of a river is determined by elevation of the drainage basin, not geographical (North, South etc.) direction.

https://en.wikipedia.org/wiki/Nile#Course