Why is mass rape essentially a 20th century phenomenon?

There are many accounts of mass rape occurring at similarly appalling rates throughout history, most notably in places like Baghdad during the Mongol sacking. Herodotus and other authors directly recorded wartime rape during antiquity. A few things to note: during several sackings, rape went hand-in-hand with looting. The 20th century, one can argue, had social structures in place that contrasted sharply with rape on a moral level. Invading armies during past eras may not have been noted as committing mass rape because it was simply accepted as part of the process, and not worth dwelling on. Up until relatively recently in history, women were considered a "spoil of war," and it isn't until (roughly) the 18th century that tangible documents protesting this sort of behavior come into existence. Furthermore, the use of media and human interest stories (since many victims of these mass rapes existed in a time when recording their stories or telling them to still-living descendants today was viable) leads to further scrutiny and knowledge of these crimes.

Edit: I should also point out that the aim of these rapes differed. Women may have been considered spoils of war by past armies, but in other cases, the intent was to actively terrorize entire ethnic groups or countries. What you noted in your question is an example of rape also being used, systematically, as a weapon of fear.