I don't think either of them has ever said why they broke up. But I could be wrong!

From interview @ living.health.com

"It’s interesting: You can love someone, but it doesn’t mean they are the person to spend your life with. I’ve been through it [in 2007, Morrison split from her fiancé, House M.D. co-star Jesse Spencer], where you think, I know I really love them, but it doesn’t necessarily make it right for either of you. [With Jesse,] nothing horrible happened. Neither of us cheated. We just weren’t right for each other."