Reality shows do so well because more than celeb acting/singing skills..people take keen interest in their lives and what's going around them, and how they lead their lives.
It's the people/viewers that make a celeb who they are.. viewers make or break careers as well. Any show that covers a complete day in a celeb's life, is loved. Ek Din Geo Ke Saath, Big Boss, Krash Landing, Lifestyles of the Rich & Famous, Kimora Life in the Fab Lane.. and many more..