In a nutshell: Google, Twitter and Facebook followed a very liberal definiton of what they are, created a platform to promote free speech to get free content from users. Being a platform, they did nopt have to edit content like print media has to. This created traffic and drove ad revenue. And everything was fine.
Then some bad stuff happened, advertisers, not users, complained by withdrawing money. And who would have guessed, that was an issue. The situation these companies re now in sucks, either they start editing (which is not censorship) and risk being treated like traditional media eliminating their competitive advantage. Or they don't and risk loosing ad revenue.
IMHO the underlying reason is that free speech and free press offline is lot easier because there are a ton of different outlet, all with a somwhet limited reach. People like Rupper Murdoch already put these limitations to a serious test. Online it's much worse with a couple of global de-facto oligopols. I wouldn't be surprised if societies and governments are moving to regulate that one day.
That being said, the fact that all these companies, leaning apparently heavy on " 100% commit to the American tradition
that prioritises free speech for democracy, not civility", are at the same time more than happy to ignore that stance in certain countries to maintain a global foot print (p.47 of the Google doc comes to mind). It seems, after all, that money is king and everything else secondary.
Admittedly, FB is much worse that Google in that regard.
EDIT: All of the above points are more or less cited from Googles own document. Except the part with e oligopol which is my own conclusion.