I apologize, I couldn't find a r/askliguists-type sub... Why didn't Koine Greek play a similar role to Vulgar Latin? Greek covered a very large area, but there aren't really different languages (with dialects of their own) that descended from it, like the Romance languages

Socio-Geo-Linguistic-Political reasons...

Tracing language rise and decline is a complex matter because there are complex factors for both (a) why speakers adopt certain languages and cease to use others, (b) how languages themselves evolve.

In the case of Greek, you have a language of exceedingly long history. Going from a collection of related dialects throughout Greece ‘proper’ and the western edge of Asia Minor, it expanded through trade and early colonization around the mediterannen basin. Its main period of expansion, though, really happened in the wake of conquest, Alexander the Great’s expansive empire lead to Greek dominance over the Eastern half of the Mediterranean, most of the Middle East (excluding the Arabian peninsula, and across to Bactria. This, coupled with the forces of Hellenisation over the next 300 years, gave Greek a linguistic hegemony that you could well see as indomitable.

In contrast, Latin expanded slowly, but also in a similar way – largely through conquest. In Latin’s case, Roman imperialism involved bringing subject groups into a Roman cultural and economic milieu where Latin-speaking was advantageous and prestigious. This largely accounts for why Gaul, Hispania, and North Africa became Latin-speaking regions. When the Romans finally headed East, they found a strong language (Greek) dominant throughout the region, they admired the Greeks culturally and adopted Greek for their upper echelons, and they tended to rule a little more indirectly, which gave space to leave the Greek language in place.

When East meets West (at least in the Mediterranean, the result was basically a linguistic divide down the middle, right about where the Balkans are. The western half of the Roman empire tended to speak Latin, the wastern half tended to speak Greek. Notable exceptions are that (a) the army tended to speak Latin, (b) the legal system even the east operated with a fair degree of Latin, and (c) upper class Romans, even from Republican times, tended to learn Greek and learn it fluently, resulting in both diglossia and bilingualism among educated Romans. This last factor was driven by sense of Greek’s cultural superiority, and Roman education patterned itself heavily on Greek models, just as did Roman literature, philosophy, and rhetoric, even as they evolved in new, Roman, directions. So then we fast-forward a little…

Greek, like all languages, underwent evolution. Its expansion led to a simplification in some respects, and the evolution of Koine Greek, or ‘common’ Greek. One of the factors influencing language ecolution is number of second-language-speakers, who tend to find it harder (hypothesized) to pick up the intricacies and nuances of a language. Greek had a lot of such speakers, and in the Hellenistic (323-30 ) and Imperial period (30 onwards) certainly seemed to simplify the language. At the same time, the loss of isolationism among Greek communities around the Balkans and Aegean also seems to lead to a loss of distinct dialects.

The picture of Greek further east is cloudy. It appears that across Persia and into India the Greek language functioned primarily at the level of government, and failed to make the ‘leap’ to common usage by the larger populace. The penetration of Greece in the Seleucid territories (old Babylonia, Mesopotamia, Syria, etc) seems to have gone ‘deeper’, but without erasing local languages like the persistent Aramaic.

Moving forward in time, Greek held a strong backwards-looking classicization element. Greek writers in Late Antiquity (say 250AD onwards) and into the Byzantine period, tended to still imitate and idolize Attic style of the 5th cent. BC. This acted as one force against language evolution, though the effect was (perhaps predictably?) an evolution towards a stronger dualism between ‘literary’ and ‘vernacular’ Greek, which eventually became the Demotic and Katharavousa split in more recent Greek.

Switching back to Latin – why and how did it survive the ‘fall’ of the (western) Roman empire? A reasonable explanation is that, as the ‘barbarian’ aka Germanic ‘hordes’ (none of these ‘’ terms are very useful or accurate) moved in and took over former Roman territories, they tended to form a small, social elite, based on a warrior-class, and to both (a) intermarry and adopt local customs and practices, (b) practice a Germanic-style kingship while emulating and preserving traces of the Roman cultural and imperial legacy. So for the new rulers of former Roman territories, Rome was still a golden legacy and Latin was still a prestige language.

At the same time, Latin survived in a ‘purer’ form being the recipient of a strong identification with the Roman church, Christianity, and the phenomenon of monasticism. In this way Latin itself bifurcated – Latin ‘proper’ evolving into its ecclesiastical strain, while Latin vernacular began taking over its adopted ruling elites. That, coupled with the fact that those independent kingdoms of Germanic tribes were less well connected to each other, gave enough space for them to develop in independent vernacular directions – the birth of early Romance languages.

Greek’s decline as a language is, again, largely extra-linguistic. The loss of control of eastern lands by Greek-speaking regimes also led to the loss of a Greek speaking elite there: Bactria, Parthia, etc.. The most serious decline, though, came with the rise of Islam and the Arab conquests. These led to rapid loss of Greek speaking lands and, subsequently, the rise of Arabic as the linguistic ‘dominator’ of those lands.

It's not always clear cut which way an invasion will go, linguistically. In some cases, the conqueror’s language comes to dominate the local inhabitants, in others, the local inhabitants’ language overtakes the conquerors. Again, numerous social factors having to do with usefulness, prestige, economics, all play a role.

The final loss of Greece itself and its survival under Ottoman rule moves me a bit beyond my own historical competencies. But suffice to say, Greek was not likely to birth any ‘daughter’ languages from that position. Daughter languages require a population large enough to sustain them, and a degree of isolation significant enough to evolve separately to the mother tongue, factors absent from Greek’s medieval history.