YokoAri is too big. The last few times were mostly graduations (2009 Elder Club, 2010 EriJunLin, 2014 Sayumi) or milestones (°C-ute 2015).
Especially now with °C-ute not participating they wouldn't be able to fill it. You can of course make fewer seats available but I guess YokoAri is too expensive to do that.