If you're feeding a pellet with treats in it, that's why. If you put a plate of broccoli and candy in front of a child, they're most likely going to eat the candy and leave the broccoli. Same thing with pellets mixed in.
Any pellet with something mixed in is not good for chins. You need to switch to an all pellet diet. Good brands include Oxbow, Mazuri, APD, Tradition, and some rabbit pellets such as Nutrena, and I forget the initials of the other one. Both Oxbow and Mazuri can be found in many pet stores and also some feed stores. The pellets with treats in them can lead to health problems.