User blog comment:Miskos3/Fairy Tail 537: The Power of Life, Review/@comment-2214433-20170601073553/@comment-7418318-20170601180804

Sales speak of popularity, not quality.

Just look at Sword Art Online. It is one of the most critizices series of all time in regards to it's quality and yet it was still a resounding commertial success (I know it's anime and not manga, but it's still a good example to illustrate my point).

And yet far better quality series have less sales due to not having similar popularity or the publishers messing up the promotions.

Granted, for long-running series the "hype of the season" factor falls away, but the "I've spend money and time on this series for years, so I'll finish it" factor comes more and more into play.