At the time of Pentarchy was established there wasn't much of a West yet. Most of what we now call the "West" both geographically and philosophically did not "exist" yet. I think that if the schism had not occured you'd probably have more partriarchs in the West. And remember the 5 is mostly symbolic anyway. All Bishops are equal in any meaningful sense.
Might you elaborate on there "not being much of a West yet"? After all, the empire had been split administratively just before 300 AD, and again, while it wasn't a literal split, it does help to explain the situation. Not to mention it appears that the cities in the Pentarchy (the 5) are either ecclesiastically, or administratively important, and there is a heavy emphasis on tying them to the administrative structure of the Empire which then comes back to the point that there were other cities besides Rome that were important - not *as* important, but important still.
These are the maps you should be looking at to see your explanation:
Thanks this makes a bit more sense. Still, that last map kind of confuses me. That's what, 500 AD, and even then (actually more so after the establishment of the Pentarchy) you have Rome as one of the 5. If I understand the point correctly, it's saying that by 500 AD, Rome itself and most of the Gothic kingdoms were under Arian control? IIRC Arianism in very large part died off, though once those groups adopted Nicene Christianity or got deposed.
Alternatively that the Eastern Empire was more stable through the tumult of the time?
You also have Carthage listed on that map (of course it fell later) and Toulouse (Roman Tolosa), A lot of those cities (especially in modern day France) would have remained Christian at least until the split, and I figure would've been closer to their neighboring areas than Rome would've been.
I'm still rather lost :s