The general pattern with any given community is that more intelligent users with better prospects in life find more productive things to do with their time. Obviously, shitposting doesn't pay, so you're going to need to dedicate your life to a career or something that'll keep you on top of your living expenses. As more people with more promise flee a given site, they leave behind frustrated, unsuccessful individuals with a tenuous grip on reality. Imageboards also change because of the next generation of edgy teenagers arrive and displace the ones who matured and moved on. Keep in mind this phenomenon is far from unprecedented. People have been bitching about the state of the internet since Usenet, hence the ominous phrase "Eternal September". There really isn't a single explanation for why every imageboard is so radically different now than they were then, but let me run this by you.
Imagine you're at the movies with your friends. You have some in-jokes and you rib each other for fun. Then, someone approaches your group with the Encyclopedia Yourfriendsica (or Encyclopedia Dramatica). This complete stranger claims to have studied the banter, pretending to be one of your crew all along. No matter how hard you try to tell him otherwise, there he stands, using all your in-jokes, often incorrectly. When someone makes a joke, this guy corrects him with the textbook definition of your in-joke and all you can do is stare at him funny. Eventually, more of them join your group and start to displace others who could've continued the spirit of your core group. Disgusted and defeated, your friends leave, and the group becomes a group of people impersonating your group of friends. The cycle continues indefinitely until the theater goes out of business because nobody wants to associate with these losers.
This is precisely why Encyclopedia Dramatica was very controversial between 2007 - 2009. That site was singlehandedly blamed for creating newfags who in turn spread their cancer just about everywhere until the moderators pushed back by banning them. Despite their best efforts, the damage had already been done. By the time sites like Know Your Meme and the like cropped up, the mainstreaming of imageboard culture was complete, and ED looked underground by comparison. Hell, bitching about newfags became tiring even among those who hated them with a passion because such conversations were decidedly unproductive.
As someone who was posting on imageboards since 2006 (Gurochan was my first IB; Death Metal loving edgelord), the old ethos was greatly shaped by the 2000s culture wars under Bush. In general, users were left-leaning liberals or right libertarian but contemptuous of any kind of liberal puritanism or progressive piety. If you complained too much about racism, sexism, muh soggy knees, et cetera, you were promptly told to fuck off. Conversely, there were a few genuine Nazis, but they were <10% of the board's population and told to fuck off back to their respective echo chambers if they got too vocal.
Then the Obama years came and the culture wars shifted dramatically. Perhaps not so coincidentally, the rise of the SJWs (who really just took progressive ideas and followed them to their logical conclusion much to the chagrin of older liberals) also influenced this new generation of users. The big black hope turned out to be little more than Bush with a fetish for insurance companies. Instead of hope and change, people got more the same. And then, the shooting of Trayvon Martin happened, and race relations take a dive, plunging to depths unseen since the 1992 Los Angeles race riots.
There was also the Libertarian-to-Alt-Right-pipeline happening with the Ron Paul voterbase, but that's a whole other can of worms.
My own theory for the Alt-Right explosion:
- A significant swath of the imageboard population are a) socially awkward, b) unsuccessful with women, c) white, d) instinctively opposed to mainstream pious liberalism, e) isolated enough from mainstream society that conventional social pressures and taboos have no effect on them. Almost all are male.
- As they get older, they grow bitter over their social failures, especially with women. This gradually turns into deep-seated misogyny and contempt for liberalism which exalts women while letting others mock socially awkward white male virgins as the last acceptable punching bag. Couple this with figures in legacy media openly shitting on people (especially men) simply for being white and you've churned up some deep-seated resentments based on identity.
- Actual Nazis appear on /new/ and /pol/. In real life, these voices are immediately shut down, but you can't be banned on imageboards for anything but illegal content, and the main demographic of 4chan is isolated from social conditioning. They're primed to accept reactionary politics: they hate women, casually use racial slurs almost daily, instinctively hate liberal society and have entered the stage of their life where men start looking for a cause to champion. They believe their disenfranchisement is a stain upon the natural order of the universe and they deserve more by virtue of being white and male.
- They're now confident and spread to other places with lax moderation. These Nazis can now hold their own when told to leave by resident liberals and even embarrass those unprepared to respond to them. At this rate, the non-Nazis leave as the percentage of Nazis grows.
- The 2016 election causes /pol/ to explode, and a whole new generation of edgy teens flock to 4chan, typically through /pol/, a link to which now appears next to the main site if you just google "4chan". They immediately adopt the /pol/ ethos, even if for most it runs no deeper as an ideology than the vague anarcho-libertarianism of 2003 4chan did among its users, but a huge swath become dedicated Nazis, and the rest at least remain open to it because it's been steadfastly normalized.
There is no single theory, but this is a good summary as to what happened between 2003 to today.