Why did Latin fragment so much?

You should probably question the premise of your question. Latin did not 'fragment so much'. It went through the expected amount of historical change for a language in its geopolitical situation.

There are several reasons why the development were not halted by the Christian canon and liturgical processes.

1. The presence of a single canon has never been enough to stop language development. As Colin Fine says - diglossia is quite a common condition. Arabic, Indic Languages, Sinitic languages. Even the many languages developing from English are all evidence of this.

2. By the time Christianity became the official religion (and certainly by the time it became dominant and much later universal), the diversionary processes had been long under way. And, of course, Christianity was not originally identified with Latin nor did much of the early intellectual ferment around it happen in Latin. So by the time Latin came to have the sacred liturgical status in the West, it was long gone as a daily spoken language the presence of which could be bolstered by religion. The processes that kept Latin so strong for so long were political and bureaucratic and it was the association of religion with power that solidified the role of Latin in Christianity, not the other way around. It is fairly well accepted that the organization of Western Christianity is the one surviving feature of Roman imperial bureaucracy that persists (albeit in trace form) even till today.

What I think would be a more interesting question is why this happens sometimes and not other times. The two languages that underwent historical change but almost no splintering are Greek and Persian. They were both the official languages of vast empires and lingua francas long after the political unity ceased. Yet, they never gave rise to new major languages (despite having ample dialects) in the same way that Latin, Arabic, and many other 'proto' languages did and English is in the process of maybe doing. I think it would be possible to trace the cultural and political forces in play that show all the nexus points where these developments were diverted (the presence of strong canons is probably not a strong factor) but they still pose an interesting counter example.