When you love someone so much that you send him straight to jail.
I mean, what better way to keep him away from other girls, while you wait till you are grown up enough to be adult and of age to be able to marry him?
Also - I've not read the source material, but I've seen more than one "harem" series wherein the one who makes the obvious jump in 'progress' first is also among the first to be cut from the race.
No clue if that sort of precedent will hold here, but it's worth noting that the youngest one also lands the first kiss (as far as I can recall anyway). Meaning the MC's obvious reservations about being pushed down and tongue-punched by a middle schooler might factor into whether or not she actually gets anywhere substantial with him.
I saw some harems where the first to fight won. Exactly cause of the fact she gone first and sparked his interest the most. In Haganai IIRC Sena was first to partially by mistake, but in the end still confess her feelings to MC that she loves him and actually would like to marry him. Didn't watch beyond 2nd season, or read the source, but I read the spoilers and if I am not mistaken she won - he chose her out of the bunch.
Also to be honest - there is a lot of harems where dude likes them all, while they behave like he has no chance at all with them. And you wait years for chapters to finally show you that they do like him, but hide it, or started liking him recently. What I mean is that harems where girls actively purse the dude aren't really that popular? Or maybe I just watched to little of them cause I procrastinate like a motherfucker lately and didn't watch any anime other than recaps on YT, and I don't read that much manga either?
On the bright side, you do eventually learn why.
Spoiler me please.
As a side note. I feel weird about this chapter. Girl is supposedly 12? And she loves dude so much she is willing to basically SA him, and is so magnanimous that she will not fuck him there on the spot and just be fine with kissing now and waiting for sex till he is ready/she is a bit older.
Me and like most of my friends at 12 - were more interested in LEGO and having fun outside. Girls as a "love interest" were just coming in, and nobody was that desperate. No clue how girls gone about that on their end. The "sex" stuff at like 13-14 - was a thing but that was basically a margin - like "bad people" without personal brakes/morals did it.