There are a lot of possible reasons, and it's very difficult to track down an absolute single reason for a social development. In this case there are a couple clear possible motivating factors: increasing information available, and reduced shame in experiencing it.

Psychiatry as a scientific field distinct from generalized philosophy or larger worldviews, and specifically the study of how war impacts the human mind, is fairly recent historically speaking. Some of that is because you get a lot of people able to travel and communicate far more quickly around the time of the industrial revolution, so a lot of fields of study get a lot more community attention and feedback. In the case of what we now term PTSD, it really only gets off the ground in WW1, but there's still a stigma attached to experiencing any sort of mental illness. It was considered weak and unmanly and shameful, most often by the sufferers themselves who didn't necessarily know what was going on or why they would experience symptoms that other soldiers in the same circumstances didn't experience. Heck, even today there's a huge stigma, and we know a lot more about mental illness than we used to.

A basic history of how we recognize that war impacts people, the various terms used to describe it, and the treatemnts prescribed: http://www.ptsd.va.gov/public/PTSD-overview/basics/history-of-ptsd-vets.asp

Over time social attitudes changed. Around the time of the Vietnam war, psychiatry was really starting to understand some stuff clearly for the first time. You can get a good laugh about the LSD and staring at goats and "if it's not one thing it's your mother", but all of human knowledge starts out with stupidity and ignorance; and the mind is wildly complicated. Scientific tools were still being defined, brain chemistry was finally starting to be mapped out a tiny bit. In the 70's and 80's we had the very first MRI's of the brain, which is a big deal. With knowledge and communication came normalization, and the beginnings of acceptance that it was ok to talk about these things openly. It goes hand in hand with a host of other social revolutions that happened around that time frame - the free and open discussion of sex was a big one. Prior to the informational revolution 60's, a lot of "Sex ed" was at home when your parents talked to you about whatever misinformation their parents from the 20's had passed on to them, and it mostly consisted of "no kissing until you're ready to have kids". Discussions of mental illness and post-combat behaviour changes were, if anything, even less detailed and less helpful.

A basic overview of the stigma surrounding mental illness: http://historycooperative.org/a-beautiful-mind-the-history-of-the-treatment-of-mental-illness/

Another good one on the stigma of mental illness: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2559930/

So the 60's and 70's were the very first time that the mental challenges associated with a war were able to be analyzed, and then socially acceptable to be talked about in semi-polite society.

Hope this helps!