Following his victory in Ravenna, Marcellus found himself an emperor. However, his Empire was divided, and facing many threats on its borders. Luckily for him, Constantine was fighting a war in Hispania, and word had not yet reached the region of Honorius' demise. Nor had it spread to the Eastern Roman Empire.
Marcellus had a very finite amount of time before he faced a three prong invasion from Constantine's rebellion, the Gothic Hordes under Alaric, and the Eastern Roman Empire. If such an event were to occur, his reign would last a matter of months.
Because of this, Marcellus was forced to choose an ally from among the three. Despite sharing a similar culture and heritage, the Eastern Roman Empire was not an option. They were ruled by Honorius' nephew, and as a member of the Theodosian Dynasty, Theodosius II would never forgive Marcellus for deposing his uncle, nor was he likely to recognize Marcellus' rule as legitimate.
