(Part of) what's great about a capital somewhere in the region of Constantinople is that it's within reach of both the Danube frontier and the eastern frontier. Putting the capital in Egypt means a central response can't reach the former quickly, which, though perhaps not a deal-breaker, is a notable disadvantage (if you're worried about Avars, Bulgars, etc.).
True but then again it depends what the ERE and it's focus is, the west or Africa and Anatolia, because you see I think it would be more worthwhile to make capital innEgypt because remember Egyp was the breadbasket of Rome and held the most manpower followed by Anatolia, so basically in the end it comes down to where Constantine wants to focus the barbaric west or the east....