A large amount of history has been derived from diaries. Were diaries more commonly kept in the past? If so, why?
A 2013 survey found that around 83 per cent of girls aged between 16 and 19 keep a personal pen-and-paper diary, compared with just 69 per cent in the 1990s. Add that to the rise of social media and digital journals, it would seem that people write more about their personal activities, feelings and relationships today than ever before.
Without looking into trends or historical data, which is always hard when it comes to a primarily female pursuit, diary writing is an activity that benefits from technology and increasing leisure time, so we should expect more diary writing over time.
Journals were originally kept by men, and only the literate minority, about significant events and spiritual matters. From the Renaissance on, people started writing about personal feelings rather than just important events. In the US, women began keeping more diaries from around 1800, and they tended to write in a style close to the modern diary - the ones men kept were more like planners. Rising literacy rates, and the invention of very thin paper allowing pocket diaries about 100 years ago, would also have made diary writing more common.
The reason why diaries are so prominent for history, is as Mark C. Wallace commented - because of survivorship bias, and because until recent history, they were the only way to find out about things like how people in the past thought and felt.