For instance, I'm currently reading Ronald Syme's Roman Revolution and I've run across this a lot. The footnotes are especially littered with untranslated passages in Latin from Dio, Tacitus, Livy, etc.
I understand that Latin/Greek used to be a bigger part of education. And, obviously, there is much to be said for reading texts in the original language. But I take a work of history of this sort to be aimed at academics (or at least historians) pretty broadly, many of whom work in areas where no competency in Latin is needed. What was the justification for not providing translations of Latin/Greek and when did it cease to be a thing.
/u/XenophonTheAthenian talked about the need to include the original language. Sole reliance on a source's original wording (meaning, no translation published) varies among publishers and journals. Generally, if the publisher expects everyone with an interest in their books as a whole will have a solid grasp of the source language, they are more likely to require book manuscripts and journal articles to use the original and not provide a translation. If a book has a wider market, they'll ask for original and translation.
An interesting case study in this is late medieval literature. Anglophone publishers will uniformly require translations of Latin and of non-English vernaculars. But they are still split about 50/50 on whether Middle English also needs to be translated (even primitive, Ancrenne Wisse-era Middle English). German-language scholarship on late medieval German literature, on the other hand, basically never translates the German. Even when it really, really ought to. (Although you can find entire modern German translations of some later medieval German texts.)
Since when is this no longer done? Any serious academic work by a classicist cites the original, not translated material. If translated material is given anywhere it's given in the body of the text and the original quoted in a footnote. One of the most serious criticisms of a classicist's work is that he worked from a translation--Badian scathingly tore apart RLF's book on Alexander for relying on the translations in the Loeb Arrian (of questionable quality), which resulted in a number of serious misreadings of the text.