Here are the titles of a few philological journals, periodicals dedicated to the study of ancient Greek and Latin literature: Philologus, Rheinisches Museum, Rivista di Filologia, Nordisk Tidsskrift for Filologi, Neue Jahrbuecher fuer Philologie, Bibliotheca Critica Nova, Wochenschrift fuer klassiche Philologie, Hermes, Classical Philology, Academy, Acta literaria societatis Rheno-Trajectinae, Eos, Hermathena, ΑΘΗΝΑ, Athenaeum, Jenaische Allgemeine Literatur-Zeitung, Philologischer Anzeiger, Jahrbuecher fuer classiche Philologie, Museum, Jenaer Literaturzeitung, Archaeologische Zeiting, Jahrbuch des Vereins von Alterthumsfreunden im Rheinlande, Journal des Savans, The Journal of Philology, Literarisches Centralblatt fuer Deutschland, Revista des Estudios Clasicos, Opuscula Philologica des katholischen Philologenvereins, Commentationes Vindebonenses, Verhandlungen der Versammlungen deutscher Philologen und Schulmaenner, Serta Harteliana, Melanges Boissier, Melanges Emile Chatelain, Wiener Studien, Litterarische Analekten, Zeitschrift fuer die Alterthumswissenschaft, Berichte ueber die Verhandlungen der koeniglich-saechsichen Gesellschaft der wissenschaft zu Leipzig, Philologische Rundschau, Eranos, and Listy filologicke a paedagogicke. Regardless of what language the title of the periodical is in, German, Latin, English, Greek or sumpin else, as late as the late 19th century it was not unusual for some or all of the articles they contained to be written in Latin. Search for these titles in an academic library or on Google Books -- entering "full preview" on Google Books will lead you to results which are mostly pre-copyright -- and you can see for yourself that this is so. Sometime between then and now it became somewhere between very unusual and unheard-of for their articles not be written in a vernacular.
Why? Darn good question, if you ask me. Let's look at a more recent copy of a philological periodical, published in 1989: Proceedings of the Cambridge Philological Society, supplementary vol. no. 15: Studies in Latin literature and its tradition: In honour of C.O. Brink.Here we find 10 articles about ancient Latin, 8 written in English and one each in French and German. And looking at the footnotes to these articles, we also find references to further journal articles and books written in English, French, German, and Italian.
All of these articles are about works of Latin. They all quote these works extensively, usually without bothering to translate anything, assuming the reader is fluent in Latin. On the rare occasions when they do translate it's when they're speculating about various possible readings. ("Tradition" in the title of this volume refers to the process by which the text has come down to us from manuscript to manuscript, with the earliest manuscripts having long since gone AWOL. The constant concern of the authors is to trace this process as exactly as possible and to attempt to reconstruct the text as closely as possible to what the ancient author originally wrote.) So why don't they just write in Latin? (Maybe they're trying to make things harder and weed out the less-brillioant would-be Classical scholars by insisting that they all are fluent in English, German, French, Italian, Spanish and preferably at least a couple more modern languages as well as the ancient ones. I would salute this deliberate weeding-out, but I don't think that's what we have here.)
What the euphemism happened here? Why doesn't anybody want to write volumes like this anymore, a commentary to Valerius Maximus, longer than the surviving work of Valerius, and it ain't short, all in Latin?Why is it so rare that a Classics professor could leave behind enough writing in Latin to fill a 500+-page volume like this one?120 years ago it wasn't odd for volumes of new writing in Latin to appear, and they weren't all written on the subject of philology, either, but also in history, geography, botany, chemistry, math and other useful topics, and not always in the form of collections of shorter works in journals. 50 years ago only the Catholic Church still produced whole volumes of new Latin. But Vatican II put the kibosh on that. Now it doesn't happen.
So you're sitting there reading this and you're saying, Hey, Steve, how about you stop bitching about this and write a book in Latin yourself? Fight the good fight and show us all the way, and stuff like that? Well, I'm working on it, okay? But I only starting studying Latin intensively after I turned 40, and unfortunately for all of us I'm not freakishly gifted linguistically like Steven Runciman. So it could take a while.
[PS, 16. July 2016: while researching this post, I discovered that the Rheinisches Museum, a philological journal, was still publishing original articles written in Latin in the late 20th century. One of several indications that Latin is, in fact, still not dead.]