That's a great question to ponder, Marduk! I don't know what conclusions you've reached from your own research, but here's my random thoughts on it...

The precise nature of the relationship between Monte Albán and Teotihuacán isn't known, but it seems to have been friendly and mutually beneficial. Monte Albán was relatively isolated compared to Teotihuacán, because of its geographical location - which may be why it never became as big or important a trading centre - however, evidence of Teotihuacáno influence in certain phases of Monte Albán's development (eg Talud-Tablero style architecture used in the Early Classic Period and the depiction of Teotihuacáno visitors on stelae around the South Platform's pyramid at Monte Albán) shows it certainly wasn't all a one-way relationship.

Though it had a Teotihuacáno presence, Monte Albán had nothing like the Barrio of course, but then - neither did anywhere else. Teotihuacán seems to have been pretty unique at that period in having specific commercial/residential districts for foreigners, so I suppose the real question is not why Monte Albán didn't, but why Teotihuacán did! *crosses eyes*. Maybe it's as simple as the fact that during its heyday "all roads led to Teotihuacán" - it was the main centre for Mesoamerica's obsidian trade and the hub of all major trading routes between the Valley of Mexico, Guatemala and the central Maya region, Veracruz and the Gulf Coast. Everyone gravitated there.

The Oaxaca Barrio is interesting because it seems to have enjoyed a great deal of autonomy, based on the Zapotec political and religious symbolism evident there, and its inhabitants maintained a distinct culture that included both Oaxaca Valley and Teotihuacán practices. But whether or not Monte Albán was actually responsible for the Barrio's presence is seriously open to question - Monte Albán did have state-sponsored settlements along the central and southern Highlands trade routes, but the Oaxaca Barrio lacks the usual ceramic markers or architectural features associated with Monte Albán and its other state-sponsored settlements. It's more likely it was originally settled by migrants from various Zapotec and related communities from the Oaxaca Valley "unofficially" moving in to take advantage of its trade links - a bit like how various foreign enclaves have formed in New York, I suppose!