…the coastal city of Ravenna, which is…

…the coastal city of Ravenna, which is protected by a ring of marshes and strong fortifications.

While the new capital is easier to defend (although in fact the city falls to opposing forces numerous times in its history), it is poorly situated to allow Roman forces to protect central Italy from the increasingly regular threat of barbarian incursions.

It is also likely that the move to Ravenna was due to the city's port and good seaborne connections to the Eastern Roman Empire.