Despite being a super short answer I find myself in agreement here the most with YE (and with PSOL's argument with ties in nicely with his). Polarization has trickled down to the forum and has made every poster here way more partisan. I think people tend to mention Kavanaugh as the turning point, though I suppose it is something that has gradually grown since 2015.
I disagree with Virginia that demographics dictate this, since conservatives on Atlas underperform even what pure demographics would dictate. I even made a thread about it
here. It is very easy to see why Atlas shouldn't be majority Republican/conservative based off of demographics, but do Atlas conservatives even reach the estimation of 30% or so? I definitely don't think 1/3 of the forum voted for Trump but I may be wrong.
Remember some demographics would also be the kind that would push Atlas to the right, like being whiter than the young US population or being supermajority male (upwards of 90%!)
I also definitely don't think something like 45% of the US population are far right, borderline neonazi white supremacists. If they are tbh Americans should all be packing their bags and moving somewhere else and I would be insanely pessimistic about the future of a country where nearly half the population is like that
(with the outcomes being dictatorship, civil war, or both)
Finally, I will note PSOL has a point too (which ties in very nicely with YE's argument about polarization). If you think about it, Atlas hasn't really become more left wing compared to say, 2012; when there were lots more maroon avatars or their predecessors, with far left rethoric and what not. It's not like Atlas is now less ideologically diverse because we have a ton of rose Twitter socialists, instead Atlas just has a ton of partisan generic Democrats (I often talk about the "red avatar blob" which is true) The modern Atlas poster isn't really more left wing, it is just way more partisanly Democratic and pro-establishment. I can think of very few left wing anti-Democratic (or skeptical of Democrats) posters (PSOL indeed being the highest profile one, a couple of maroon avatars, but that is it).