After the dissolution of the Roman Empire in the West, Latin (albeit not a Latin that the Classical authors such as Cicero, Caesar or Vergil would have recognised as fit for literary purposes) was one thing that survived. The Church in the West by then had a tradition of Christian Latin going back 300 years. The vernaculars - the languages spoken by ordinary people - emerged only later; & the literatures in them later still.
IOW, there were good historical reasons for the predominance of Latin in (much of) Western Europe.
Latin also had the prestige of a sacred language, as it had been used for the title over the Cross.
And Catholicism is a tradition-minded religion - so the Vulgate, which had been rather shockingly modern at first, having displaced other versions in Latin older than itself, managed to become traditional as time passed - so that by the time of the earliest fragments of Old French, it was 440 years old; older than the AV-KJV & the Challoner-Reims Bibles are now. So by the time of the earliest fragments of the Bible in French, it was even older. So most Western vernaculars had, by contrast, no status at all in comparison.
Even now, we do a lot of our thinking in Latinised English - which is why it is possible to read of the “porrection of the instruments” The catechist I mentioned this to, refused to believe I hadn’t invented it - it is Latin English for the “extension of the instruments” (that is, of the paten & chalice) to the newly-ordained priest in the rite of Ordination as it was before 1944.
Latin is necessary to read the learned literature of the years before 1700 or so. Newton’s Principia, published in 1687, was in Latin. Milton wrote in Latin as well as in his own tongue; so did Calvin. As for Catholic theologians, they can’t be understood without Latin - this is as true for those writing in 1940 as for those of 490. Without Latin, a very great deal of the intellectual work of the past is a sealed book.
I’m all in favour of pure, jargon-free, Latin - there is no reason why it cannot be used today.
Si res novas inquiris, spectate illic http://ephemeris.alcuinus.net/