Adam, admit it, you posted speculations as fact.
For all we know Darcey could have had all sorts of personal reasons to quit, nothing to do with the show.
She said she had other commitments. She probably wants to concentrate on promoting ballet and dance in general. Strictly was always a loose fit as far as her experience and interests go so it makes sense. She's done her bit, cemented her profile and off she goes to do something more in line with her goals.