I'm wondering if it was all just too much for her.
After all, it wasn't even her decision to join. Her mum put in her application for the MM。audition. She probably didn't have any dreams or ambitions of being an idol in the same way that others might. And being so young, she probably wouldn't have realised just how much work it is.
Between the fast major debut with all the events, the H!P concerts, being opening acts for so many things and all the travelling, perhaps she just couldn't handle it. Maybe she talked to her family about it and her family talked to UF*, but in the end no conclusion could be reached, or she realised she'd just had enough.
I'm sure being an idol isn't easy, and it's a lot more work than people think it is. But the thing is, it's the sort of thing that's almost impossible to know if you're able to handle doing it or not until you try. And I'm sure she did want to try and do it and continue -- but she couldn't. She always looked happy and sparkly on stage, so I doubt she was being forced to do anything against her will. I think it was more an internal thing and she couldn't cope with it. Perhaps it was the pressure, perhaps it was interacting with all the fans -- we'll probably never know. but there are so many aspects to it and I'm sure for some girls it's just far too overwhelming, especially at a young age.
I'm really sad about this, but in the end, her mental health and wellbeing are the most important things. And if she couldn't handle being an idol, then I completely understand (that's basically my entire life, except on a much smaller scale) and I respect the decision. I wouldn't want her to suffer or be forced to stay if she's going to be stressed all the time. And it could potentially lead to so many worse things down the line.