Why wasn't she expelled sooner?

Well, Clare Short has finally got around to handing in her resignation (again...)
This time, she has again fallen short of going the whole hog, and only resigned the whip. Although given her voting record, I suspect the whips office barely expected her to follow the Labour troops through the same lobby.
But given her outspoken comments, I'm surprised that the PLP hadn't expelled her sooner. Most of the attacks on her have been because she has said she wants to see a hung parliament, which implies that some of her Labour colleagues will have to lose their seats. As Labour has continually lost seats since 1997, this is hardly beyond the realm of possiblity! But normally a hanging matter in the Labour party. So why did the chief whip hang Short out dry sooner?