Why did Isobel Campbell leave Belle & Sebastian?

I realize this was five years ago but after listening to a bunch of albums last night, my curiosity was piqued. Merci.

She initially offered the conventional response: personal reasons. But she later elaborated that she was more interested in pursuing a solo career. Here's what she said during an interview:

"I left Belle and Sebastian because I just wanted to explore different kinds of music -- to mill around and do my own thing."