Volume sales doesn't really matter when it comes to a manga getting a anime adaptation, "100-man no Inochi no Ue ni Ore wa Tatteiru" from Bessatsu Shonen Magazine is having a anime airing now, reportedly improving the manga sales but the last volume released on September (a month before the anime) didn't even appeared in the Oricon Top 50 with 5 days, on Shoseki did "17,308", less than Yozakura #5, and this is only 1 example from the current season. Shueisha/SJ don't have to wait for any studio to be "interested", if they want to, they can just fund it themselves and hire a studio, also no one here really knows about what is going on behind the scenes, anime adaptations can be (reportedly) decided even before the first volume of a series is released, so as we speak now, maybe a Yozakura anime is already decided or even in production and we just don't know yet, same applies to Mashle or Undead.