'Cause Disney made it that way?
No seriously, I *think* it was really intended to be a discount on table services restaurants for the frequent visitor - that's why it's only for AP holders and FL residents. The places that it is accepted for CS meals are just places that (at least at one time) did not have a TS location -- SSR, Animal Kingdom, and the value food courts.
So the better question might be - why are any CS locations even included? But that's why they are.