She's been in the group for almost 5 years, so, like, that's a LOT longer than most KSS. Besides, I dunno, I'm surprised she lasted this long since she wasn't getting promoted.
Not being chosen for MM or S/mileage is probably the main reason why she decided to quit now after all these years. How many audition rejections can you take if you see several newer kks being chosen to debut almost straight after joining.