I'd say it's the nature of medieval society. By its very nature most polities in Eurasia were multilingual and thus could easily get away with certain languages used for different purposes. This is the age of Galician poetry in Spain and Provencal-speaking trobadours elsewhere, of a Catholic Church that spoke in Latin, of a time where the King could speak in one language and the peasantry spoke another, though most early polities here were based more on loyalty to the monarch rather than anything else. Just as how in the modern UK you still have people who speak Celtic languages or Scots despite the huge all-pervasive presence of English (yes, I know!), then so too would it be in the case of French in England - which is not to say French did not have much of an impact, since artifacts were preserved both in Middle English and the traditional pronunciation of Latin in England (and hence how our original Latin, Norman French, and general Romance vocab is pronounced in English in a very Anglicized way) - with Celtic and Germanic languages still spoken by the people in much the same way. It's a bit similar to how we can approach Old English - we now know that Germanic tribes who migrated westward were few in demographic terms, but that early English society was a hybrid of a majority Celtic/Roman populace with the Germanic minority, and hence in this matter Old English could be viewed in the same way, IMO. It was a minority language much like Norman French in the post-Conquest period, but it did receive a lot of impact from the substrata - an early form of hybridity similar to the formation of Scottish Standard English, essentially. The spread of English in what would become the modern UK and, in part, Ireland was not so much an imposition from above but was a more informal, gradual approach based on trade, which is part of the reason why English is not uniform, alongside other reasons in other languages as to why regional, social, etc. variation exists in the first place.