The work that the girls get is often determined by popularity and seniority. But when it comes to these radio shows, I have a feeling age is a big factor. In Japan, you can't work after 9 pm if you're younger than 18. Risa and Eri, for example, are 18 so there's no problem for them to have a show that airs after 11 pm. Yeah, I know some of these shows are often prerecorded, but there could still be some scheduling problems, especially with Youngtown where the girls probably have to follow Sanma's weird schedule, which might be late into the evening. Sayu is turning 18 in a month so it will be less of a problem using her, whereas with Reina, she won't turn 18 til November. When that finally happens, I wouldn't be surprised if they end DokiMiki and give the timeslot to Reina.