I've gotten one reply to my query that the Latin is in part because the RC chuch, historically didn't encourage individuals to read scripture themselves, but rather intended such things to come solely from the clergy. I'll need to research that idea further before I accept it or reject it, but it is food for thought, and it is a logically consistant answer.
My reply to your question was in #283.
If you MEANT to ask why the Church began using Latin exclusively around the year 600 AD, it was because Gregory the Great (Pope) sought to unify the liturgical practice of the Church.
And BTW, don't allow someone to feed you that garbage about "only the clergy understood latin." Remember that before books, people actually knew by memory anything important--Jews could recite enormous chunks of the Torah, and Greeks could recite enormous chunks of the Odyssey.
The "lived in year 700, therefore stupid and illiterate" theory is only a product of evolutionary theory plus an enormous amount of current-day arrogance.
Dubyaismypresident already gave some good reasons in post 276, but some historical context might help as well. In A.D. 500, the Latin Mass was a vernacular Mass, because most western Christians spoke Vulgar Latin. And then you had the barbarian invasions, and with them came languages like Gothic, etc. Gothic translations of the Bible were in fact made (one famous one by Wulfilas). But as those barbarians Romanized, Latin still remained the language of law and scholarship, and religion as well. Then gradually, as you said, Vulgar Latin in the various regions began the shift to Italian, French, Spanish, etc. So it wasn't so much that the Roman Church *decided* to have an archaic language at its heart, it's more that the vernacular language it had always used slipped out of usage everywhere else.
On the clergy not encouraging people to read the Bible part--that's a mistaken characterization. Vernacular bibles were always available, but before printing were fantastically expensive and nothing your average peasant Christian could afford (if they could read at all). Thanks to Gutenberg they were more available, but during the Reformation the Church got really touchy about bad translations getting propagated and falling into people's hands. That's where the greater restrictions came in, but I believe it pertained more to people *printing* and translating Bibles than owning them, which has always been encouraged.
Here's an interesting little tibdit--not spot-on the discussion, but it will be helpful:
http://www.catholicculture.org/docs/doc_view.cfm?recnum=4749