In general, I've seen a few standard explanations for this:
- The problem is social media - It is something about online that makes the discourse toxic and unproductive. Platforms like Twitter and Facebook facilitate conversations in a way that incentivizes harassment, attention-seeking, sociopathy, and so on. Their moderation is either non-existent or actively malicious; their engagement systems (favs, likes, and such) create compulsive behavior; quirks of the medium (character limits, the permanence of casual posts, the absence of tone) are uniformly hostile to friendly, productive dialogue.
- The problem is some guys - In any given left space, there are a handful of bad apples who are ruining everything for everyone. Among this local DSA chapter, it's some problematic dude or clique who dominates conversations, who are trying to consolidate power, who have caused yet another controversy; online, it's the weirdo pariah who constantly sows conflict; at the level of national politics, it's Clinton, or it's Bernie.
- The problem is first-world privilege - First-world activists aren't actually dealing with significant political problems; their endless scandals and controversies are just what happens when people who are used to getting their way socialize and encounter even the smallest disagreements. If the first-world "left" actually faced significant oppression from the right, it would show much more unity and solidarity than it does.
While all of these things probably contribute to the general atmosphere of combat and hostility, I want to propose a simpler answer that I think is too often overlooked: the problem is anti-socialism.
Even today, I do not think there is an adequate appreciation on the left of just how marginalized and reviled socialists are in the United States. Historically, socialists have usually been regarded - by Republicans and Democrats alike - as dangerous, immoral, and alien extremists. We have routinely been denied basic civil and human rights, both informally (through selective enforcement, de facto discrimination, etcetera) and formally (anti-communist legislation, private sector employment policies, and so on). We are the target of an entire discourse of bigotry with its own arsenal of slurs (EG pinko, commie, brocialist). To put public sentiment in perspective, here's a typical poll from less than three years ago:
For most of my life as an activist, my experience of anti-socialism has reflected this poll. When I was involved in SDS, my chapter was surveilled and attacked by police officers; we were routinely ridiculed in the media; we were shut out of supposedly "public" spaces and denied "public" resources; and quite often, our most ferocious and belligerent critics were Democrats who identified as "left" and "progressive". Organizing against the Iraq War, the left organizations I was involved in could often only count on solidarity and coordination with local Muslim groups; even when Democrats adopted an antiwar posture around 2005, we were still marginalized as extremists, apologists for terrorism, and unpatriotic traitors. During the early Obama years, you couldn't even call for a public option - much less for single-payer - without being attacked by liberals as unserious, privileged idealists who were willing to risk the health of women and minorities in pursuit of a pipe dream. This is how it's always been.
And while I doubt that the left wants to hear this, I don't think all that much has changed. Today, the most favorable polls say that only about a third of all Americans have a positive view of socialism - a fifteen point improvement from 2009, but still lower than Trump's lowest approval ratings. And when you break down the numbers*, the picture gets even clearer:
On one hand, even the minimal support that socialism has is squishy: put four people with a "favorable" view of socialism in a room, and only one of them will support it without reservation. On the other hand, meanwhile, opposition to socialism is still rabid. A plurality of the opposition - and in fact, a plurality of the entire population - doesn't just dislike socialism: they hate it. On balance, then, the picture is clear: a tiny fraction of the population will embrace socialism without qualification, while an overwhelming majority meets it with tepid support or open hostility.
In light of this, I think there's some solid quantitative evidence to support the timeless, enduring explanation that socialists have always offered for left factionalism: liberal squishes. A significant number of people who are willing to nominally identify as socialists are in fact deeply suspicious of the socialist project; either unwittingly or quite cynically, their hope is to co-opt socialism for liberalism, to steer it back towards liberal priorities, and to purge its ranks of anyone who is focused on the fight against capitalism.
Account for squishes, and a lot of the other explanations diminish in importance. It's true that social media has made left in-fighting more visible than it's ever been before, it's true that a few doofuses are disproportionately responsible for all kinds of local and even national dysfunctions, and it's true that a lot of these controversies emerge precisely because the political stakes are so low. But anti-socialism remains one of the most powerful forces in American politics, and what would be surprising is if it didn't wreak a massive amount of havoc on the left.
* Unfortunately the polling on public attitudes towards socialism is still really spotty, so it's hard to get a clear view of how attitudes have changed, especially over the past few years, and especially if you want to break down the data.