Why are popular articles unfit to use?
Popular articles (those published in popular/general interest magazines) are not unfit for use. They can be great sources if you're only looking for general information about a topic and don't require a lot of detail or in-depth analysis. Keep in mind that articles in popular/general interest magazines are usually written by journalists, and not scholars in a given field.
Before you use popular sources for your assignments, double-check the requirements you've been given by your professor. Often professors will require that you use at least some scholarly sources.