IMO/IME this is not exactly how it happens. What really happens is that as these platforms grow, they attract the attention of religious zealot groups who conduct absolutely massive campaigns to take down any content that they personally see as inappropriate. They have their own private blogs and mailing lists, with people given names/emails to contact to lodge complaints about "highly offensive" content that they have never even viewed, they were just instructed to file complaints against it.
A big part of the issue is that much of this content really only appeals to a small group of people, and many times those people do not even realize these attacks are happening in the background. The content platforms then decide it is easier to just ban this content so that they do not get overwhelmed with the religious zealot protests.
I wish we could charge those kinds of groups with the impediment of free speech (at least in the US) and fine them out of existence.