It started with the radicalization of the right starting in the 70s (more explosively in the 80s and 90s), and now has expanded with the radicalization of the left in the past decade or so.
The basis of the radicalization of the right started mostly with the Powell Memo. This set the stage for conservative think tanks whose purpose was to rationalize that all things that support corporations are good, and anything that doesn't directly support them (via increased profits) must be bad. The American Enterprise Institute utilized the concepts espoused in the Powell Memo and shifted from a technocratic neutral organization into a staunch RW mouthpiece, and other groups were soon founded such as The Heritage Foundation. Once Fox News was founded, the masses had 24/7 constant spin to feed RW ideals to them...whether they understood that or not.
As the right become increasingly radicalized over the decades (the fact that there are
any defenders / rationalizers of Nazism clearly shows how horribly far right they have become), the left decided to start up their own counter-approach with the development of Huffington Post (sorry to break the news to the RW idiots, but MSNBC, NY Times, Wash Post, etc are not left-wing...that is another black-and-white fallacy pushed out by RW sources).
With the advent of the internet age, the RW noise machine moved in quickly since they were already completely in place. The LW noise machine, not be left out at this point, has greatly accelerated and developed their own equivalents so that there seems to be as much stupidity on each side with sites such as Fox News and Huffington Post being marginal, and outright garbage sites such as The Daily Caller, Breitbart, Infowars, The Blaze, RedState, David Wolfe, Occupy Democrats, U.S. Uncut, Addicting Info, and Natural News.
Additionally, much of the mainstream media is simply satisfied with covering the drama and not providing any substance to the issues anymore. This is the true crime of CNN, MSNBC, ABC News, NBC News, etc...they rarely provide actual journalism (e.g. discussing in depth what the recent health care bills actually did or did not do) instead providing sports-like broadcasts (e.g. discussing whether the Republicans can get the necessary votes). This has left a major gap for most people who must either rely on their partisan news or actually spend lots of time reading much more robust articles from sites like Vox, Atlantic, Economist, Fiscal Times, etc (which the vast majority of people are too lazy to do).
Thus we have what we see today. Two groups of people who think they know the truth because their partisan sources tell them what that truth is, but lack the intelligence to actually discern what is really going on. Remember that even the dumbest person thinks they are above average, so without a good source of information, they will continue to think their stupid ideas are actually at least better than the average persons ideas.