Strategically, the main difference between western and eastern Romania is the Mediterranean Sea : while the first have (and fails) to defend a long border with Barbaricum and especially the vulnerable region of Pannonia (which was significantly underdevelloped compared to other provinces, and still required a lot of manpower to fill the gaps), which couldn't be protected everywhere at the same time (for instance, Stilicho focused on protecting Pannonia, whom neglect could threaten Italy directly, and the crossing of the Rhine in 407 was essentially met with Frankish foedi).

Comparatively, eastern Romania was similarily touched by this but essentially in its European parts, in Balkans. It happened that the main opponent of Rome, Sassanians (Barbarians rather playing second roles up to the Vth), stopped pressuring the eastern border at this point : Hepthalite raiding in Persia, a growingly powerful aristocracy, and a general focus on inner matters made eastern Romania essentially spared to face a double front (safe two small scale wars).
Eventually, as Constantinople kept control of the seas, thanks to its fleet and fortyfing key strategical positions, eastern emperors simply didn't have to deal with Barbarians nearly as much as their western counterparts had to. (One of the main reasons why western emperors choose Ravenna rather than Milan in the Vth was that Ravenna, surrounded by swamps, couldn't be taken without fleet)

Constantinople managed to keep control of its provinces for the most part, even in Europe (foedi still existed, of course, particularily along the Danube) while western Romania felt prey to warlordism as imperial authority was unable to ensure its defense, would it be Barbarian (trough foedi as Goths, or self-imposed kingdoms as Vandals) and Roman alike (Aegidius in Gaul, Marcellinus in Illyricum, etc.). Simply said, as Barbarians got more and more involved into military matters, to make piece for a declining quality and avaibility of Roman army (whom best elements were tempted to just join with Barbarians), they became more important as a political factor whom western emperors and generalissimos were unable to get rid (would it be only to beat up other Barbarians, such as Aetius did against Huns) and still recognized as breaking away the empire (Majorian's wars against foedi mostly searched to beat them into submission, rather than crushing them).

Then, it meant that eastern Romania had mostly intact fiscal and logistical capacities (Greece and Syria even showing signs of demographical growth), when only Roman Africa was really spared and represented most of fiscal and supply of the western imperial state before the middle of the Vth century) : it represented such a loss, which led to coastal raiding and loss of western fleet, that it can easily be seen as the point of no-return for the western empire.

In the end, it was too much in a same time for the WRE, while ERE had mostly intact capacities. The situation in the 450-470 does gives an interesting highlight, with Italo-Roman elites essentially content to defend Italy and roughly accepting the loss of control in provinces if it was the price to pay (altough Ricimer still tried to intervene in northern Gaul in the mid 460' but it was concluded by setting up Breton garrisons instead of Alans), while eastern Romania decidely focused elsewhere and didn't trust any barely legitimate authority in Ravenne or Rome

Sources

- Barbarian Migrations and the Roman West, Guy Halsall
- Late Roman Warlords, Penny McGeorge
- Rome and Barbarians, Peter Heather