Publishing-house magaziens have insisted on their positions to critisize giant newspapers. They also think it their mission to report the theme that national papers do not cover. Newspapers put importance on the objectivity. Weeklies, on the contrary, stick to subjective, cynical view points. In a sense, their method is a kind of scandalism.
Newspapers take advantage of their big organization and privileges such as press club sysytems.
The reason can be traced back to the starting period of these magazines when Japan was so poor that people wanted to find cheap entertainment that could satisfy most of their interests. The large circulations of these publications prove the concept still meet Japanese tastes, maybe. See an experimantal chronology . You may want to visit homepage of The Japan Magazine Publishers Association . It's a good source of information, but very heavy in its graphics.