sophie
Break My Heart
Constantinople is much further away from Gaul than Rome was. And I thought Rome had a good defensible position? Or are saying that Rome was vulnerable in a general geostrategic sense (which wouldn't make sense either, given where it was in Italy)?
Strategically Rome was extremely difficult to defend. It's in a low-lying region without any natural defensive positions to work off of. Building a wall entails building an actual wall around the city, and on top of that the city was huge. The Aurelian walls were built at great expense, and even then, it's difficult to cover all the positions at once. This is leaving aside the further point that Rome by the 3rd century was pretty much entirely reliant on grain from Africa, meaning not only would a captain defending Rome have to worry about covering all points of the city simultaneously, he also had to worry about key trade up the Tiber getting cut off.
As Dachs notes in his post (he knows far more about this stuff than me; listen to him!), the later stages of the Roman empire revolved around the relationship between the Emperor (patron) and the nobility (client). The patron provided the client with patronage in the form of favorable governmental positions, authority, and prestige, and the clients in turn granted stability, legitimacy, and authority to the patron. By the later Empire the Gallic nobility had supplanted the Italian one as the most powerful (and therefore most important to maintaining this relationship) clients in the Empire. The capital was moved north (to Ravenna and later present-day Milan) because it enabled the Emperor to better carry out his responsibilities in the patron-client relationship, as the whole system was built on the physical presence of the Emperor for the thing to work. The Empire broke down, not because "rahr Barbarians", but because this patron-client system collapsed. The Western Empire destabalized (due to rancorous civil war and a not insignificant stretch of horrendous luck) to such an extent that the Emperor was no longer capable of adequately providing the Gallic nobility with patronage, and so, the Gallic nobility found new patrons who could.
In short, Rome ceased to be the Roman capital because it ceased to be the locus of Roman affairs. A bit tautological, sure, but there you have it. If you're really interested, as with illram, I can't recommend any book more highly than Halsall's. He covers the stuff I just said in far more detail than I could ever hope to match, and in a far more eloquent manner to boot. (What are you still doing here? Don't read this schlock! Go buy the book already! Leave!)