She didn't belong here anymore. She didn't want to be a hero just yet, all she wanted was to gain control and return to her studies. But, life at the mansion wasn't giving her anything. The Professor had promised to help her with her control, but since she'd arrived there had always been one crisis or another that kept his attention on other things.
Before her mutant she'd had a promising life, she'd been accepted to some very good graduate school, not that she's shared this information with anyone. Why expound her differences from others her age. But now, after her experiences, she's decided that her dreams deserved some attention. She'd gain control by herself, while going to school.
The Cure, and the battle at Alcatraz came at just the right time, they'd never even think twice about why she left. They'd assume that she'd left for the cure, and went on with her life. When it came time to return to the X-Men she would, but not before. She could feel that there were other things she could be doing with her life, her dreams. Then after sometime she could go back to playing hero.