I guess it's kind of like having to study the Q'uran in Arabic if one wants to say one has really studied it. That's considered the appropriate and sacred language for it.
I have to wonder how the Reformation, and indeed the Renaissance, would have progressed if not for the widespread influence of the English Bible. It would have been a different world, certainly.
Latin is still wonderful for musical settings, like Vivaldi's "Gloria" in D minor. It wouldn't be the same piece in English. A cultural notion of what constitutes sacred language makes something in that language "feel" more sacred, as if you're singing or speaking what has been spoken by many before you.