Well, if the majority of candidates are applying to a job where they only meet four out of five of the requirements, if the employer can add a sixth requirement they may naively think then applicants will have five out of six requirements. Alternatively, if they receive too many applications, a solution is to be more specific so they receive fewer or they can filter out more earlier. Adding additional requirements is one way to do this, even if the requirements are not necessarily connected to a successful candidate (knowing how to write in languages that aren't used in the company, for example); some recruiters don't seem to know that some of those requirements are completely irrelevant to the position.