Why are Quantic Dream Sony exclusive?

Quantic Dream, a studio you may know for its creation of Fahrenheit, Heavy Rain and the upcoming Beyond: Two Souls (featuring DAFOE!!) and also an unannounced PS4 title, is a strange company. Why am I calling them strange? Is it that they make unique games? Is it that they focus on emotion and characters? Is it the French accents? No to all, I am calling them strange because they are exclusive to Sony.