CommentStreams:5b69e288fb413f58cc1e0d59c5598dd9

"Mirai Akiyoshi was under 18 at the time"

Then that's the reason, from the looks of things: https://dmm-corp.com/press/corporate/337/

I believe the following part of their statement is talking about that topic:

"しかし、昨今の業界全体の流れを受け、イメージビデオ取扱い基準の見直しをし、18歳未満が出演する商品は、たとえ性的な描写がない場合でも取扱いの停止を独自に進めている最中でした。

偶然このような形で当社が取り上げられることとなりましたので、上記取り組みの進行を急遽早める形で、平成28年9月7日に18歳未満が出演する作品の取扱いを全て停止いたしました。"

Basically, DMM.com announced in early September 2016 (as in the month before this was supposed to be released) that it would stop selling/distributing anything involving people under the age of 18, even if it wasn't explicitly pornographic. Maybe Spice Visual either got scared or a deal fell through with DMM.com in regards to distribution, or they thought they wouldn't make enough money via DVD sales alone. I'm not sure.

Mirai was only 16, and her birthdate was public knowledge.