The school thing again. When someone leaves a group or gets out of show business to continue their education, I don'think they believe they will make it. They need to be certain they will have a job a couple years from now instead of being forgotten idol. So, I don't blame some of them, but they usually leave early, like Asuka. Give the band a chance before you leave.
Although, I don't know why Mizuho broke up ZONE to leave, though. It already established a strong hold over the industry. She still wants to continue her education. I guess some people would choose an education and a normal life over all the stress of stardom.