Reasons why BOYS AND MEN sold 40,450 copies on first day (4 times of Juice=Juice's sales), despite being a relatively unknown group
1. It's one of their better songs, fangirls love it
2. It is the opening song of the drama "Shiratori Reiko de Gozaimasu"
I think the following is the biggest factor
3. They have a special event "Head petting event" (I don't know how to translate that better lol)
http://www.kingrecords.co.jp/cs/t/t7333/ with preorder of the special version. Basically fangirls get a pet on their head with handshake

and also, regular events such as handshakes and 2 shots.
Their previous single "BOYMEN NINJA" released last month had a shocking first day sales of 86,905 copies (even I was really shocked by the amount).
They had single release events almost every weekend, promoting single sales with handshakes and 2 shots.
I have no idea why I decided to write such a long post about them ._.