Why was transportation on the Nile easy for trade and unification?

"Egypt, the gift of the Nile"
(I suggest to watch the following video for a lesson on the matter:
Ancient Egypt: The Gift of The Nile )

Because the river Nile, a very long river, crosses the Sahara Desert, almost directly from the Southern Nubia and Aethiopia to the North arriving at the Mediterranean Sea, and because the rise (floodings or inundations starting soon after Syrius was seen rising just before the Sun, the annual coming of the god Hapi) and fall of the river level left fertile sediments in its margins, many human communities formed close by and along the river.

The first cataract (rapids) in the site where is now the Assuan Dam, protected the Upper Kingdom, because large ships could only travel the last 650 miles of the river Nile. And the Delta marshes didn't offered harbors for invasors. The strategic position of the Nile, amidst the Sahara Desert, protected the Nile Valley from invasion by land.

Beyond the first cataract, the second one (or Great Cataract) is much upstream, in the former Nubia, and is now submerged under Lake Nasser.
https://en.wikipedia.org/wiki/Cataracts_of_the_Nile

So the river is very navigable, and even more so because there was scarce need to row since the river flow pushed (pushes) the ships Northwards and the winds mostly from the North pushed (pushes) a ship's sails Southwards. In its navigable stretches the river isn't shallow and doesn't have many dangerous rocks in its riverbed. So, as the most practical means of transportation it was the perfect way of contact, trade and integration of the human communities along the river.

The Delta Region meant many entrances to Egypt, sea fishery and contact and trade with other countries such as Crete and Phoenicia.

It was just a question of time for appearing a group (such as the Menes') that was able to submit one community after other along the river without fear of external intervention.