It started as a deliberate engineering technique. Let's take the most left wing guy we can find, and make him look like an evil crazy weak moron fascist pants shitting wimp robot that nobody likes, by sheer force of peer pressure and insult. It hurt Al Gore and John Kerry quite a bit, and I think the Dean Scream was probably the peak of the middle school peer pressure bullshit. I think soon after that a lot of people just stopped paying attention to the TV news for anything. But it's still happening; now it's just phrased as things like "Biden is old and out of touch and we're concerned that his polls are down." I think around the time of Bernie Sanders (when no one gave a shit that they were broadcasting that he was officially not cool and a crazy person communist stupid head and everyone better stop liking him), was when they realized they'd have to regroup and come up with a fresh updated strategy with it.

Also, the beast has gotten out of control, a little bit -- it used to be exclusively a tool of the corporate media aimed at the lefty-est candidate, but it started being how the political press covered everything, to the point that they couldn't really steer it anymore to shit exclusively on the left wing candidates they were trying to destroy. Then a while after that, Trump came along and was 10 times better at it than anything the news could come up with, and the rest is history.