I think it happened due to simple money making incentives. 1. Media companies make more money with more viewers, because they get more ad impressions. 2. The most successful newsmakers make content that gets more viewers to advance their promotions 3. They use stuff that pull at human instincts, other news makers see it's effectiveness and start copying it. 4. repeat

Once you've worked in large organizations for a while, you realize that humanity most of the time is way too disorganized and gives too little of a shit for any kind of "conspiracy" to be that coherent.