It has been hinted a few times that Maipuru lacks drive and determination. Before she joined Idoling!!!, she was having trouble deciding what she really wanted to do with her life, and her decision to audition for Idoling!!! was basically based on a whim. One of the reasons why she's so multi-talented is because she's the kind of person who will take interest in something for a bit and then suddenly quit when she loses interest. Knowing this, I'm not surprised that Rurika is getting the solo push now instead of Maipuru. I'd even say that there's a small chance that Maipuru actually turned down the opportunity for a solo push, seeing how she had no real solo activities after that one concert last year.