Apparently because quite a few people have had bad experiences with it? When filesystems go bad, they really go bad, so it makes sense that these users would feel motivated to share their experiences.

Perhaps it's a vocal minority, but does it matter? Isn't the error rate what's most important? I've seen ZFS horror stories too, but not nearly as many, and that says something at least.

Disclaimer: I've never used btrfs. I quite like ZFS.