or the conquerors weren't enough (or not brutal enough) to enforce the spread of their own language; e.g. the Franks conquered Vulgar Latin-speaking France, but they adopted the Latin-based language instead of making everyone else speak a Germanic language.

You're forgetting an important aspect: It was cool to be Roman back then. The people who conquered the Western Empire were people who wanted to be part of Roman citizens but were refused by the xenophobia of the late Western Empire (not to mention the whole christianization thing), and would essentially larp as Roman for a bit, so it's not like they were in a rush to assert ethnic dominance. "Barbarian" peoples conquering centers of civilization and assimilating into them rather than vice versa is a pretty common thing in history, see: the Mongols. This as opposed to early Muslim Arabs, who rode their own religion and culture to regional dominance and would have them spread across their conquered territories instead.

while the middle east was almost fully (muslimized? islamized? there's got to be a word for that).

"Islamized" is the word.