Because it's the longest river,and ancient Egyptians used it.
Your question is not clear as to what you want to know, so the link below will give overall information.
(nile) is greek for longest -Nikki Gallagher
Nile is from the Greek Neilos - meaning "river valley"
The Ancient Egyptian word 'neilos' meaning valley so maybe they named the river after the valley it brought life to.