For one thing, because threads about BLM protesters and cars didn't blow up like this one with 2000+ comments. For another, Dang judged - probably correctly -

For one thing, because threads about BLM protesters and cars didn't blow up like this one with 2000+ comments. For another, Dang judged - probably correctly - that a large fraction of people active on HN thoroughly hated Kirk. Without the warning, he might have needed to ban a lot of active and useful contributors, and he'd rather not need to do that.