1)Silicon Valley surrendered the high ground in terms of free speech. In the past, Silicon Valley at least used to preach a near absolute commitment to free speech. Now, they don't anymore.
2) Consolidation of content. Before, the internet was very distributed. Number of nights a 1000 poeple might regularly visit could be in the thousands. Now, a significant number of people just visit Facebook, Twitter, YouTube, or one of a maybe a dozen top sites. This makes enforcing regulations much easier.
3) Silicon Valley managed to pull off the near impossible feat of angering both sides of the political spectrum at the same time. The left was angered by the perception that the tech companies', especially Facebook's lackluster enforcement of policies allowed Donald Trump to be selected President. The right was angered by the apparent enforcement of political correctness at the expense of free speech. In addition, it became clear with this election that many in silicon valley despised the Right. Thus, Silicon Valley was left with few defenders.
4) Governments realize how disruptive tech can be to entrenched power structures. Things like the Arab Spring, Brexit, Donald Trump's election would have been much more difficult without the Internet and social media in partiy. The powerful across the gloves in China, Russia, EU, and now the United States have realized that an uncontrolled Internet can be a huge danger to their ongoing status.
Put all these together and it is very likely that this is just the first wave of further regulation of Silicon Valley.