I see three factors.
The amount of disposable money available to youth (teens-25) has increased.
Society has changed from family based to individual based.
Religion has decreased as an influence in peoples lives.
Popular music speaks to the people who buy it. (If a song doesn't speak to the people, they don't buy it and it doesn't become a popular song.)
The "voice" of a modern song is younger than the "voice" of more classic country. Compare the "age" of the singer in What Was I Thinking with that of Good Hearted Woman. "She snuck out one night" vs. "she'll welcome him home once again" What Was I Thinking is about young love (aka lust), Good Hearted Woman is about an established longer term love.
On top of this, society has changed so the youth is disconnected from the working life of their family. Today's prime demographic doesn't have have the work/life experience to relate to songs like Picking Time or 5 Feet High and Rising. The current notion is that teens aren't supposed to work, teens are supposed to school and party. Rather than teens are supposed to help their parents on the farm. This changes the themes that speak to the paying audience. Even songs like One Piece At A Time speak more to the working folk than to the partying folk.
The loss of religion as a theme applies not only the explicitly religious songs, like Daniel Pray, Lonesome Valley and I Saw The Light, but also to the background faith in songs like Keep On The SunnySide and Family Circle.
The historical themes that you are looking for can be found in classic country like the Skillet Licker's - Run, ******, Run (which encourages a slave to escape the slave catchers). (I wish I remembered the name of the Carter Family song about an ex-slave witting a letter.) Songs like No More Grey or Blue were also a part of country when those who had experienced that life were still alive. Social change and justice have been a part of country. Johnny Cash's Ballad of Ira Hays, Loretta Lynn's The Pill and One's On The Way for example. Hank Jr.'s Keep The Change is in that tradition.
The historical story telling songs come from the roots of country/folk/blues. Which is why they aren't played today. Today, its all about copyright and new songs. When was the last time a current artist put out an old standard. Terry Clark's - Poor Poor Pitiful Me? Sawyer Brown - The Race Is On?