It’s possible that the producers (or whoever) realized that the online marketing isn’t really helping the sales anymore. I’ve been regularly stalking the BD sales, and despite the lack of tweet (and Nonaka art preview) for Vol. 10, it’s BD sales were hardly different from Vol. 9.
Vol. 8 - 1,284
Vol. 9 - 1,187
Vol. 10 - 1,275
Granted, it’s already really low, but that’s for a different topic. I’ll get back here if they released the numbers for Vol. 11, though I won’t be surprised if that’s also the case for this.