Two theories of change:

(1) An increasing number of Americans see free speech as a positive rather than negative right. In other words, “Facebook must host whatever I want to say”, not just “The government can’t arrest me for insulting the president.” That’s a controversial expansion of the right to free speech, creating a lot of debate and disagreement.

(2) The internet means Americans are more exposed to non-Americans. At least some of those people come from countries where free speech is seen as a right that should be balanced against other rights, like privacy. That makes it seem like fewer people support free speech than in the past.