From Wikipedia (
http://en.wikipedia.org/wiki/Sotsugyou):
To date, the only sotsugyous from Hello! Project entirely have been original Morning Musume members Asuka Fukuda, and Aya Ishiguro, second generation Morning Musume member Sayaka Ichii, fifth generation Morning Musume member Asami Konno, solo artists Michiyo Heike (reportedly the only unamicable split from the H!P family), and Rika Ishii, Coconuts Musume/MiniMoni member Mika Todd, and original Berryz Koubou member Maiha Ishimura.
That made me think...maybe she left because she wasn't getting the attention she deserved at the original contest winner. I mean even from the start she got overshadowed by Morning Musume, T&C Bomber, and eventually Melon Kinen-bi. Early H!M episodes prove that quite well. If that "unamicable split from the H!P family" stuff is true, I think I have just given the reason why just now :-X...poor Micchan.
I just realized that even Heike's nickname was replaced! Mitsui Aika now don't the "Micchan" nickname in H!P...double :-X.
Oh well, I wish her luck in her solo career. She deserves it!! I loved "One Room Natsu no Koi Monogatari"!!