My theory on the first day sales:
AKB48 was already majoring focused on after the #1 Weekly Oricon Chart for RIVER. And so it wouldn't surprise me if some random people on the street bought the new single just to see what AKB48 was about, especially with the news coverage from RIVER; the younger generation would probably notice the numerous cute girls (like most other idol groups); the older generation might draw back on the days of Onyanko Club and wonder what the hype is with Aki-P's latest idol group.
After that, there's the different genres that the single offers: the graduation song 'Sakura no Shiori', the song to rock out to 'Majisuka Rock n' Roll', the song with the "idol" feel 'Enkyori Poster' and the upbeat but sad song 'Choose Me!'; each catering to a large variety of potential buyers. Each song also has its own reason for drawing people in:
- March/April is graduation season in Japan, which is a rather big thing. It signifies that you're moving past the 'student' stage of your life and becoming a true 'adult'. For those people, 'Sakura no Shiori' may feel more important as a song.
- 'Majisuka Gakuen' is a drama in which idols become yankii-fied. How often do you see that?! Plus, the timeslot places it as a relatively optimal showing time, considering it's being shown overnight on Fridays. I mean, after a long week at work, what better way is there to kick back and relax than watching idols slur their words and beat each other up? Alright, I might be exaggerating there, but the rarity of the situation still stands. And as the opening theme song of such a rarity, we get 'Majisuka Rock n' Roll'. 'Cuz I'm sure that whoever has watched an episode of 'Majisuka Gakuen' can at least hum parts of 'Majisuka Rock n' Roll' (assuming he/she doesn't skip the theme songs).
- Finally, Weekly Playboy and Young Jump are popular magazines. And the contest between Team PB and Team YJ has been advertised for quite some time now. I mean, it's basically a joint event between Weekly Playboy and Young Jump if you think about it. And the winner gets to do gravure for both magazines! Even if you have no clue about the contest, you'd still notice the (usually) bikini-clad girls on the pages and that might grab someone's interest. And seeing those girls again singing and dancing a song written just for them may lead to buying that particular single (maybe?).
I'm not gonna go into the details with the handshake/2shot events ('cuz I have no idea about them to begin with..) but I think I've laid out a few reasons for the popularity of the new single. Of course, I doubt anyone could have predicted such an increase of sales between singles (clearing RIVER's total sales in the span of 24 hours? That's unbelievable) and I really don't care, to be honest. I'm just glad that the girls are doing well and hope they continue to do so.