The values in the culture changed. My hypothesis to explain how and why it happened is this - the idea of seriously devaluing lyrics took root in the south sometime during the 2000s and really exploded among the culture in the last 3 or so years
My theory for why it exploded is the increase of non-lyrical rappers producing mega hit-records. For the culture to demand music like Fetty Wap, Future, and Thugger to the degree that it currently does, the condition of lyrics being a high regarded value had to change, since the real popular artist don't meet that metric.
As the culture morphs to accept that type of music it puts a constraint on rappers who depend on bars to draw, which is why I predict a artist like David East will never become big as a Future, and why teens from certain black sub-cultures will never give a fukk about a Nas.