Mainly that a) it wasn't really favorably oriented to deal with the Empire's problems (i.e. it was too far away from Gaul and the northern borders), and b) it had the Senate. Also it was totally reliant on trade with other regions for supply, it was an impossibly hard position to defend, and the city was in a state of disrepair by the 3rd century.
Most of this, yes.
Over the course of the third and fourth centuries, the monarchy became even more of an overtly military state than it had been before. The change was less in terms of iconography - Augustus had never pretended to be anything other than a dictator whose power rested on his army - and more in terms of management and control. The Emperors, especially in the West, found it expedient to maintain permanent residences close to zones of military operations, partly for propagandistic purposes but mostly so they could directly manage those military operations.
In addition, the West saw something of a shift in regional authority during that period. By the late third century, Gallic provincial leaders were easily as powerful as those in Italy, who basically lost their monopoly on power. Moving the capital to Gaul and Germania had the salient effect of improving the Emperors' abilities to manage the Gallic aristocracy. The proximity of the court and the expansion of the army meant more opportunities for patronage there, which kept the locals happy. The consequences of
not massaging Gallic interests were frequently dire. In the 250s and 260s, Gaul raised up a series of contestants to the imperial throne when Gallic magnates felt that they weren't getting enough of a slice of the patronage pie; the same thing happened beginning in the 380s after the Emperor Gratianus returned his capital to Italy.
It's harder to articulate why the capital did not return to Rome (with one or two brief exceptions) after Gratianus flipped his Gallic supporters off. It's true that managing the Senate was a factor, but only in a negative sense: in the fourth and fifth centuries, it was exceedingly possible to manage the senatorial aristocracy without actually residing in Rome, something that was probably
not possible in the time of Augustus. But that doesn't really provide a positive reason for the Emperors to live in swampy fortresses like Ravenna. Defensibility might've been one concern, but the problem with that is that there weren't really military threats in Italy until long after the move was actually made. Avoiding the Senate might've been a factor, but the Senate generally lacked the capacity to actually interfere in politics during that period. It's true that the city of Rome was often "in a state of disrepair" at points in the third and fifth centuries, but it's hard to see whether that was cause or
effect of the Emperors' departure.
of Qin
