NOTE: I’ve noticed many people linking to this as a source of information. While obvious, do remember that I am not a primary source for any of this. Rather, this information was pieced together from what we randoms believe we know about Jump‘s rankings. If that’s what you intend to link to this for, link away.
As I’ve said previously, Shueisha’s Weekly Shōnen Jump is currently the most popular manga magazine by far. Even the flops sell better than the high-sellers of many other magazines. As a result, they have the leverage to keep the magazine fresh by kicking out 2-3 of the worst performers every ten or so issues. Poor performers are measured by reader surveys, and volume sales if they’re high enough.
Some series survive due to extremely high volume sales, despite being ranked the lowest in the magazines (an example being To Love-Ru). Other series sell poorly and survive on ranking alone (an example being Hinomaru Zumō). Living through even a single full year in the magazine is fairly difficult. Not only that, it’s practically a death sentence for a writer’s career in the magazine if their series get cancelled three times, not that it’s easy to get a slot after being cancelled twice.