Upvote:7
Why? Because successful ruling elites try to increase their wealth and power. Ancient Egypt was mainly arranged along the Nile with inhospitable desert to both East and West. This leaves only two directions for expansion. Egyptian polities expanded along both of them with the following results:
Of course, we may go deeper and ask why this happened, why this happened when it happened, and who was responsible.
Sources
There are three instructive articles in the Journal of Archaeological Research on the matter in the last 20 years:
Reasons
The literature discusses various causes:
... and many other causes of various importance.
The Narmer palate
...should not be overestimated. Although an important historical document because of the lack of other such documents from the same time (besides Pharaoh Scorpion's mace head), this was a relatively insignificant object at the time. It shows various things (the use of sophisticated writing, the existence of the institution of the Pharaoh, the importance of some ideology involving the unification of what later became known as upper and lower Egypt since Narmer is shown wearing the crowns of upper and lower Egypt respectively in different scenes), but there are many things that it does not show beyond reasonable doubt: Did the crowns Narmer was shown wearing actually stand for the whole of upper and lower Egypt at the time? Was Narmer the one responsible for their unification? Etc. etc.
There are many things about the process of the unification of Ancient Egypt that we do not understand. We should be very careful when interpreting the sources ...