The sites of ancient Mesopotamian civilizations tend to be very dry, unlike those of ancient Egypt. Looking at the areas via online maps also shows, that while the Nile river basin remains very green, the areas around the Tigris and Euphrates look pretty dry (admittedly this might not be the best metric).
I tried to find more information on this, and while it seems that government programs in the last century contributed to desertification in Iraq, clearly the phenomenon is considerably older. Another statement I have seen repeated is that desertification occurred on account of soil salination “due to millennia of agricultural exploitation”. Admittedly Mesopotamia has seen large-scale agriculture before Egypt — but in total seems to have remained suited for agriculture for a shorter time.
What's the difference then?