The problem has been a long time coming, effectively since the inception of televised news broadcasting. What we're seeing now is just a gross acceleration of it. Once news corporations included advertising in their broadcasts, and their businesses were run as a business and not a public service, most were willing to bend stories and gin up headlines to catch more and more viewers to increase their bottom line. Thus the birth of the 24 hour news cycle. The only real way to keep people watching when you're saying the same thing over and over is to make it as explosive and exciting as possible, most often at the expense of accuracy.

The other problem has been going on even longer. People like to have their own beliefs reaffirmed; we've been like this since we first showed up on the planet. Now, however, targeted advertising (which fake news rides on the backs of) only makes it easier to get caught in a loop. Before it was fringe groups that would say the mainstream media was all made up/coordinated to hide things/conspiracy conspiracy etc. But now we have a president elect and high ranking officials who claim the same, loudly and repeatedly, and it's only helping to erode things further and much more quickly.