Yozakura is not a bestseller. It sells less than 100,000 per volume. It's a great manga, it's had success in other areas, but it's not a big seller.Shonen sirius has Yozakura Quartet, one Snk spin off and they use the magazine to make manga adaptations of anime.
Those are the type of series Betsumag probably see as their best case scenario.On Monthly Shonen Magazine they have a few mid hits.
Mashiro no Oto
Noragami
Shigatsu wa Kimi no Uso
Not on magazines. Manga magazines have been declining for decades. Shingeki fans might be willing to spend a lot on memorabilia, the majority probably never buy the magazines, only tankobon. Even with a circulation boost, Betsumag is still a small, specialty magazine.For Japanese fans, they are willing to spent tons of money for their favorites than trying new things.
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Isayama, Inoue, UrasawaWho in Kodansha is bigger than Mashima. I thought he's one of biggest name in Kodansha already.
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Well, Clamp is. but they're busy writing Gate7 and xxxholic Rei.

It's worth noting that Fairy Tail is not Kodansha's best selling manga. Just off the top of my head Shingeki, St. Young Men, Vagabond, and Uchuu Kyoudai all outsell Fairy Tail. There's a whole bunch that sell almost as much as Fairy Tail. Series like Giant Killing, Billy Bat, and Say I Love You sell less, but not a lot less.