I don't know why they split, I suspect that we'll never really know, no that that'll stop the wild speculation about it.
I've simply always felt that Cressida, from what little we know of her, would find a royal life stifling. Someone whose education and career prospects have been in dance is unlikely, in my view, to find a career cutting ribbons and shaking hands somewhat stifling.
She has her own particular style, seems to have something of a 'free spirit' about her, she is her own person. I can imagine her looking at the life that'll be expected of her and the constant criticism of every minute aspect of Kate's person - her clothes, her hair, her posture, her hand movements, her facial expressions etc ad infinitum, and just feeling that sort of life would not suit her. I know Harry's not exactly the picture perfect prince, but royal life is a cage at the end of the day, a fancy shiny cage, but a cage nonetheless.
More power to Cressida if that's what she's decided.