Because they’ve discovered that trolls and jackasses in comment threads actively work against people taking science seriously.
Per this earlier discussion of comments, I think Popular Science is probably doing the right thing. The site doesn’t benefit from hosting dis-and-misinformers, and such folks are becoming more persistent and possibly more organized. Best to punt them entirely. They have their own places to stew; let them stew there. In the meantime the information in the articles will speak for itself.