I think there are many things in play here.
Firstly the trend from a studied portrait style to a more photojournalistic/documentary style was happening before digital came into being. In my opinion it started when automatic/autofocus cameras hit the scene. People were ready for a less formal looking type of photography and the style permeated everything from fashion to downtown portrait /wedding studios and it caught on.
So formal studio type knowledge of lighting was slowly being replaced with available light and many looked like little more that better printed snapshots.
When digital hit the scene and every Tom, Dickhead and Harriet now has a camera, photoshop-like post processing and then cellphone picture manipulation, this informal, lack of technique snapshot became the norm. I have no idea where it's heading but since people do become bored, I think that more formal lighting styles will become again recognized and appreciated.
The problem is that once there were 50 good photographers in a city of 100,000, now there are 50,000, and they have the sheer power in numbers to dictate where the trends are.
I'm not even sure if analog people really realize the massive shift in the professional market that happened because of digital, and how the game has changed so incredibly. They see that everyone has a digital camera and marginalized their preferred capture/print medium, but in reality everything, I mean everything, about photography and professionally photography has changed, from the style, to the market for selling it.
As for taking years to master lighting, if you look at cinematographers who work in major motion pictures, with massive budgets, a need for perfect lighting and no room for fuckups, you will find that almost all are white haired old gentlemen doing the work.