i believe (i don't know enough about this) a big factor is also how your peers treat you after you experience trauma. think about veterans coming home, friends and family don't understands what they went through. they can't talk to anyone because nobody takes them seriously, even to the point of disbelief, or they blow things out of proportion. maybe the veteran doesn't feel any pain, but they tell him that he should. either way, the veteran is completely misunderstood. (again, this is not backed by any knowledge of psychology, just a guess based on some personal experience)
so because in the past everyone experienced the same brutal life (except the land lords o otherwise well off), it wasn't as traumatic as it would be today.
On the individual level… for me, certainly, the hardest part of dealing with my particular flavour of living nightmare was that literally nobody could relate to it, and the others who went through it are scattered to the winds. When you explain something painful and the response is “I would have considered myself lucky to be in your situation” or words to the effect, it’s a rather lonely thing.
In the past, if you said “the baron came and murdered my child on a whim”, people would likely sympathise - in today’s society… well. We have such a buffet of lives and possibilities that comprehension of the worlds of others is increasingly challenging.
Solzhenitsyn put it well - can a man who is freezing understand one who is starving?
Even in areas that were some distance from the towers there was a relatively high incidence. They put it down to a background level of stress including sources like violence on TV.
This was in comparison to other parts of the world that were less modern and less busy than experienced more traumatic events but had less PTSD. The conclusion was that if you generally have a more boring/ quiet life you will be better able to absorb trauma if it occurs.
What makes you think this wasn't common?
Basically, the common argument is that life WAS emphatically much more brutal in the preindustrial past, and people felt the same grief that we do today — however, such events were less likely to serve as an etiology of chronic anxiety-centered conditions like PTSD for a multitude of reasons. E.g. you can’t lay around depressed in bed all day if you rely on subsistence agriculture because you’re going to slowly and painfully starve, or you’re exposed and desensitized to brutality from a younger age, or you have less strict social requirements/expectations than a modern 9-5 would impose so you can grieve and act out on your own terms, etc.
Dan Carlin’s “Painfotainment” is a good self-contained intro to these ideas for a casual audience, if it seems up your alley.