Guess being in a J-idol group has become just "something to pass the time while waiting to enter university" instead of a career choice?
Everyone knows that with the nature of how things are in the Japanese entertainment industry, idols have a "temporary shelf life" at best. The waves of popularity inevitably cycle/change as new, younger idols continuously enter the scene, thus (for lack of a better expression) pushing aside those who have been around for a while.
It has only been recently that idols have been able to have idol-related "careers" longer than, say 4 or 5 years on a fairly regular/consistent basis, which, if anything, makes them the exception to what has traditionally been the norm. Based on current trends, it would appear that being part of larger groups like H!P and/or the *48 conglomerate have helped in that regard, however, in the end, being an idol never really is a realistic, long-term "career" choice for these girls. Many idols don't get the widespread/national popularity that they may initially be looking for, which can be disheartening/discouraging. If an idol is fortunate enough to get that popularity, then they have to deal with the grind of continuous, sometimes seemingly non-stop work schedules (concerts, TV appearances, etc.) that inevitably just wear them down/tire them out. And as is usually the case, as much as they may love being idols, at some point many of them eventually get new goals/dreams that they want to reach and acheive.
It has been said before and it'll be said again, although this (the inevitability of idol graduations) sucks for the fans, we do realize that in the end, she's doing what she feels is best for her, and it probably is the smart/better move in the long run.
Now with that being said, time for the obligatory fanboy reaction: