I'm not sure "failed" is quite the right word. There are plenty of forums still thriving in specific niches. But yeah, they definitely lost their position as the dominant form of online community.
Could fragmentation be the reason? In forums, every topic has its own little island with separate accounts, different sites/rules/interfaces. Compare that to just posting in a subreddit or Facebook group where you're already logged in. Switching websites requires mental work that social media doesn't.
