Well over half of all web traffic is from cell phones, and the mobile-first web is hostile to long text content. It is also incredibly hostile to user-created long text content.
With the exception of people who need keyboards for work (tech workers, academics, journalists, writers, etc) there's no one left online to make community content anymore. Hence, HN (a community for tech workers and similarly minded people) and Twitter (where most organic, non-public-relations major players are journalists, academics, or writers).
What passes for web communities have almost entirely become playgrounds for intellectual lightweights to spew memes at each other combined with a platform for entities posting inorganic content that pretends to be genuine.