A scholarly source goes through a process of peer review. Books are selected by editors and then sent out to readers who are experts in a given field. Journal articles are likewise sent out to expert readers in a field before they are published.
While popular sources don't have this same standard of rigor, there's still expectations in quality and relevence that need to be met. An editor still decides whether or not a book or an article will be published, but the criteria that go into the decision popular press editors make is sometimes different. First, popular sources have at least an ostensible goal of making money and provinding some form of entertainment; they are directed toward a broader audience than academia.