I think a few broad trends reinforced each other in the U.S. But kind of prerequisite, people came to get too much of their notion of free speech as a scholastic doctrine in state schools. Mills's On Liberty argues that a doctrine elevated above criticism comes to be held in an empty way, and gets lost over time even if it's true. It seems like that happened here to the idea of free speech itself.

(I'd quibble at your "completely destroyed". We don't want to see what completely destroyed will look like.)