People make these claims about HN based on their own political passions, which lead to false feelings of generality. If you had the opposite politics, you'd notice opposite things and derive opposite conclusions [1]. Plenty of people do:

"leftist totalitarianism prevalent on HN" https://news.ycombinator.com/item?id=20202305

"any opinion that's not 100% politically correct && strictly SJW standard compliant is suppressed" https://news.ycombinator.com/item?id=17197581

"anti-western and extremely anti-capitalist" https://news.ycombinator.com/item?id=20384662

"biased to the left" https://news.ycombinator.com/item?id=21325122

"quickly turning into a leftist SJW and socialist haven" https://news.ycombinator.com/item?id=23351311

"always politically left" https://news.ycombinator.com/item?id=23396632

"Obviously this website is rigged for the liberal agenda" https://news.ycombinator.com/item?id=23429442

"skews quite left" https://news.ycombinator.com/item?id=23661802

"a liberal echo chamber" https://news.ycombinator.com/item?id=23658461

"They purge anything that doesn't trend left leaning" https://news.ycombinator.com/item?id=23808107

"Marxism and thought police are rampant here" https://news.ycombinator.com/item?id=23986096

"HN is ran by radical leftists, so no surprise" https://news.ycombinator.com/item?id=24037475

"if you say anything that doesn't align with the mainstream liberal consensus you'll be flagged and a mod will reprimand you for flaimbait" https://news.ycombinator.com/item?id=24353254

"You aren't allowed to go against the propaganda arm of the Democratic Party here" https://news.ycombinator.com/item?id=24782327

"anything critical of the Democratic party is instantly flagged [...] And the moderators don't give a fuck either because they're also in the main HN demographic of SV white liberal trash" https://news.ycombinator.com/item?id=24990311

"They purge anything that doesn't trend left leaning" https://news.ycombinator.com/item?id=23808107

[1] I think it's simply because we're more likely to remember those: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...