At the end of the centuries long Pax Romana-(or "Roman Peace"; around the 200's AD /CE), Germanic tribes from Northern and Central Europe-(referred to as, "barbarians" by the Romans and Greeks........who invented the word), began to invade the Italian peninsula with numerous fighters. Germanic tribes, such as the Goths, Visigoths, Ostrogoths, Vandals, Teutons, Franks, Lombards, Angles and Saxons proliferated throughout Roman colonial Europe, primarily into Italy, as well as pouring into the Roman colonies of Northern and Western Europe.
The lands of Ancient Northern and Western Europe were primitive and largely undeveloped-(except for the well built Roman roads). There were few cities North and West of the Alps during the Roman Empire. London-(originally known as, "Londonium"), was a swampy marshland during Roman times, though towns, such as York in Northern England, as well as Trier, in Western Germany, were more sophisticated and developed-(as one can see from the preservation of its Roman archaeological sites). However, the majority of Northern and Western Europe during the Roman colonial age, was, (when compared with the Eastern half of the empire), poorer, less developed and culturally unsophisticated. This part of Europe were the badlands and wilderness of the Roman Empire-(with FEW notable exceptions).
The pastoral nature of Ancient Northern and Western Europe was a perfect spot for the Germanic tribes to settle into and conquer. Although there was the impressively built Hadrian's Wall in the North of England, it would do little to protect the Roman colonial West from Germanic militaristic onslaughts elsewhere. When compared with the Eastern half of the Roman Empire, the Western half was largely unprotected and unguarded, due to poor funding, as well as disregarding its overall geopolitical value to the Empire.
As for Italy proper, the North of Italy, during Roman times, was mostly, a primeval, forested wilderness-(with the notable exceptions of Verona, Assisi and Milan.....which became the Defacto Capital of the Roman Empire beginning with Constantine). The rich Latin speaking cities of Italy proper, were, Rome, Ostia, Tivoli and before its volcanic eruption, Pompeii, as well as nearby Herculaneum. The historically Greek cities of Southern Italy and Sicily, such as Taormina, Syracuse and Naples, were also rich Latin speaking cities during the heyday of the Roman Empire. If you were an ethnic Roman living during and after the Pax Romana, you would have much preferred Rome, as well as the cities and towns to its South, rather than the rural villages directly to its North.
But even the affluence of Central and Southern Italy during the Roman era, was waning when compared with the towns and cities to its East..........namely, the Greek East.
Emperor Constantine-(and even his predecessor, Diocletian), "saw the writing on the wall". Diocletian and especially, Constantine, recognized that the future of the Roman Empire was to the East of Rome. Although Milan would become the new administrative capital of the Western Roman Empire, it was the centuries old city of Byzantium that was of growing important strategic value.
The Germanic tribes, were primarily a landlocked peoples who had little or any familiarity with the sea; their invasions throughout continental Europe were led by primitive armies, though not by sophisticated navies. In other words, it was the strategic presence of waterways that Constantine and his generation recognized in Byzantium. The city was at the end of Europe, though directly across from Asia, as well as within reasonable sailing distance from Alexandria, Egypt-(the starting point of Africa). The control of Byzantium meant the control of the Black Sea, as well as the nearby Mediterranean Sea. Having such control would lead to commercial and strategic advantages that few other cities around the world had; one could trade via land, but also by sea.
The strategic and commercial value of Byzantium under Constantine, was a redirection of affluence and power from Rome proper.