Ichiko was a convenient side affect of Mizuho living in his mother's old room. Regardless of Ichiko, they said that it was in his grandfather's will that he go to that school as a girl, and live in that specific room (that Mizuho later found out was his mother's). In the episode where Ichiko "ascends" (eps 5) Mizuho realizes that Ichiko's Onee-sama was his mother. He then goes on a breif speech about why his grandfather made him go to that school and live in that room. Since Mizuho only knew his mother from a hospital bed, he was sent to the girl's school to get to know his mother better. There's not much detail other than that, but it's pretty blatant.

If that's what you mean by not explaining it in detail, then you're probably right. They've explained every aspect of how Mizuho came into his particular situation.