Mostly he founded Constantinople as the western territories were contributing much less money to the Empire than the east, at least by 300 AD. It was a shift so that the capital would be the centre of the wealth again.
Communis opinio is, that miltary needs were the major reason. Diocletian and others already resided in Nicomedia most of the time. Because the Bosporus is located perfectly between the two major fronts of these days: Lower Danube and Euphrat. An emperor had to lead campaigns himself these days in order to legitimize himself in front of the army. Rome was simply too far away from the major fronts.
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