Question

Why is the delta region to the north called Lower Eygpt

Answer 1

That area is called lower Egypt because it is physically lower than the rest of Egypt. That's also why the Nile River flows to the North.

Answer 2

The terminology "Upper" and "Lower" derives from the flow of the Nile from the highlands of East Africa northwards to the Mediterranean Sea, the opposite of a north-to-south flowing river like the Mississippi. Lower Egypt mostly consists of the Nile Delta.