I really doubt that any one reason explains the political makeup of a given site, but what really drives things to the extreme is turning into an echo chamber.
Sites usually start out with a mix of views (unless they're explicitly geared toward a certain ideology, obviously). That mix is almost never perfectly even, though. For one thing, there is no population on Earth with an even mix of different worldviews, because the history and culture of any given society is going to shape what's considered mainstream and what's on the fringe. This leads to a global population that's uneven, since some ideas hold broader cross-cultural appeal than others, and some societies just have more people. There aren't an equal number of Christians and Jains on Earth, or an equal number of Neoliberals and Marxist-Leninist-Maoists. You'd have to deliberately work to create an even ideological spread, and even then some groups would absolutely refuse to participate based on opposition to that concept. An Independent Fundamental Baptist isn't likely to agree to disagree with an LGBT activist for the sake of dialogue, so you're going to end up with a lot more users close to the Center and on the Libertarian Left than on the Right or the Authoritarian Left.
In practice, though, any given website (other than the few social media sites large enough to absorb competitors like a supermassive black hole) will be even more biased than the population in the region or regions it draws from, because users are self-selected even at the earliest stages. To get back to Something Awful, it's an old forum, so its initial userbase were Internet early-adopters, who tended to be wealthier and better educated white Westerners. Especially at the time, those demographic traits correlated heavily with Center-Left to Libertarian Far-Left politics, with a smaller segment of Right-Libertarians. While they were important to the early history of the Internet, with a lot of the founders of early Silicon Valley companies coming from that political demographic, they were too small to serve as a meaningful counterweight, so SA started out leaning toward the Left.
As a site grows, the early majority or plurality almost gets to decide both the etiquette used in conversations and the range of views that are considered acceptable, unless the owners dictate those, which is pretty much always unpopular. It's a concept called an Overton Window in political science. For a site designed for the general Internet-using public from the time that SA was founded, that window would usually start out with its center skewed significantly further to the Left and Libertarian sides of the Left/Right and Libertarian/Authoritarian political axes than the population in general. Over time, though, what usually happens on forums, just in general, is a sort of slow bend toward the direction of the original userbase's center of gravity. That trend definitely holds true for the SA forums. First, people far from the community's initial center point realize that they'll be banned outright if they express their opinions, and sometimes even if they're well-known for expressing them elsewhere. Then, people on the fringes of acceptability see that they'll shunned unless they leave or keep their views to themselves. Eventually, only opinions close to that center are regularly posted, changing the tone of any and all political discussion on the site and driving away new users who might find it offensive, argumentative, degenerate, retarded, racist, or [insert nasty adjective here]. Old users leave, get banned, or sometimes literally die off in the case of a forum as old as Something Awful. New users expand the dominance of the largest political group, and shift the window farther in its direction, sometimes to the extent that the old center is, itself, outside of acceptability. You see that in its most extreme form on the large Marxist sub reddits, which had a sizeable founder population of tech-savvy Tankies looking to find a new outlet, away from the Libertarian Socialist dominance of most IRL Leftist groups in the mid- to late-2000s. Talk shit about Stalin in 2011, and you'd get into a heated argument about kulaks, where about 50% of the participants were pro-Gulag morons. By 2025, having a single post on a sub where someone had once allegedly compared Xi Jinping and Donald Trump to Winnie the Pooh and Tigger (without having a bomb mailed to their house, of course), will get you an autoban with an appeals process that requires you to know how to suck moderator dick like an 80 year old veteran whore called Blowjob Betty, who never bothered with dentures because she already knew how to chew better with her tongue. That's if you don't want to get laughed at and called a Fascist when they tell you "No", of course. That appeal's going nowhere.
There's also a real-world element when it comes to Leftist echo chambers. From the 1990s through roughly 2012, Leftists often used dark, edgy humor to piss off their Evangelical Christian political opponents. That could include jokes about Nazism, lynchings, sexual assault, etc. As Internet echo chambers have gotten larger, though, they've begun to have an impact on real world politics, driving both sides farther toward what were the margins a little over a decade ago, and creating a push-pull dynamic between the two that has been especially beneficial for the growth of the Far Right. When you're on the Left and your opponents include a small but highly visible proportion of actual Fascists, jokes about how Hitler did nothing wrong just hit different. This had the effect of changing the tone on forums like SA to a more serious version of what it has been, where serious racism, misogyny, etc., would get you banned, to one where it was first seen as too close to Alt-Right meta-irony (ie., saying shit you actually believe in an exaggerated way), then offensive regardless of how obvious the satirical intent might be.
The Left has gotten entirely too dry, serious, and full of itself, while the Right can sometimes actually meme. As dumb as it sounds, I feel like that's a part of why younger generations are leaning in that direction. Not the whole reason, obviously, but telling people that they can't say anything that could offend anyone under any circumstances (unless they're a political opponent, of course) just sounds fucking stupid to everyone on the outside looking in.
