Who killed Latin, Part I: Charlemagne
The theory goes like this. Prior to the Carolingian Renaissance of the eighth and ninth centuries, speakers in the former Roman world weren't conscious of the fact that they were speaking differently from one another, or that their pronunciation varied wildly from what was written down. The reason why is simple: most of the Romance-speaking world prior to the reign of Charlemagne was illiterate. Those very few who could still read and write just pronounced the few documents they had in their possession in their vernacular, dimly unaware of the growing frequency of glosses required to read older texts.
By analogy, think of the many words in English that were locked into their spelling during the transition from middle to modern English and before the great vowel shift. Here's a short list: through, thought, rough, of, two, live, live, read, read, great, eat, head, etc. These are are English words, but can be confusing to learners of English because their pronunciation varies wildly and inconsistently from their spelling. Still, we speak English, and these are English words, everybody knows that. By writing 'of' and saying [uv] I'm not being inconsistent, I'm just adhering to an orthography standard.
To a Romance scribe in late antiquity, the archaic spellings may have been felt the same way. A scribe living in Spain might write 'habeo' and pronounce [e] and think little of it.
Then literary rose during the Carolingian period, and the court reformers were very keen on restoring the language to its proper form. During this period, many texts were 'fixed'. That is, where 'i' was written 'e' or vice versa, or h dropped or added, the reformers worked diligently to bring the texts up to full Latin snuff, even going so far as to removing new grammatical features that were now a part of Romance but hadn't existed in the classical language, and uplifting grammatical forms that were long obsolete, like the dative and ablative.
The reasons I've heard as to why this was done are these: 1) Incorrectly said prayers couldn't be understood by God, so getting the correct spelling and pronunciation were important if the Franks were to continue to be in the Grace of God, 2) Scholars of this period genuinely loved the classical language and thought it beautiful and wanted to 'raise the bar' as it were to restore Latin to it prior glory, and 3) A standard of writing across the realm would aid people who had to read it, instead of guessing at what someone on the other side of Alps was actually saying (this explanation dovetails well with their other writing reform, standardized Carolingian miniscule).
If people weren't aware that there was a difference between then and now, it was readily apparent. Within two centuries, scribes unknown to history started writing down what their spoke as different than what they read and write. Latin becomes a de facto lingua franca across the Romance-speaking world--just as the reformers would have hoped--but is understood to be a written language only. Shortly after the Carolingian reforms, the Franks start speaking French.
Did an act of love on the behalf of Frankish kill Latin by making everyone so acutely aware of the differences between Latin and their native dialect? Charlemagne is the first suspect in this murder mystery, ironic as history seems to indicate that his own Latin was poor and he was probably illiterate.
The next suspect is Romance itself. Stay tuned.