It's explained in CBL.
But in a nutshell it's because they are slow digesting, and will be in your system for a longer period of time, so while your body is busy trying to digest low GI carbs it is not losing fat as the fat loss hormones are inhibited.
Plus it will not cause the insulin spike required to transport the nutrients/proteins into the muscles.
The science behind it is explained in the book over several pages.
By the way I have not tried CBL myself yet, still making great gains on my current low carb diet, without any backloading.
Once I get closer to my goals will give back loading a try and will monitor progress very very closely.