The numbers of electrodes is not the only thing that affects sound quality. The article mentions that it can be inserted deeper. Keep in mind that the distal end of the cochlea is where high frequencies are produced. Beyond 3kHz there isn't much speach perception going on. Most speech perception is done between 750hz and 3Khz.
More hearing devices (whether hearing aid or CI or anything), since limited in power and signal processing capabilities, have focused on improving speech based communications. Improving sound at 12kHz may be nice for someone into music, but it comes at a high cost (battery life, electronics size, etc) for a feature that is not used by many people or very often.
C1