I think we need to remember that they are still making money on the magazines. Many have suggested that since the donation arrangment came in they could be getting paid twice for the same magazine. Firstly when the witlless picks up the mags then secondly when it is placed on the door.
I can't see that combining them would save much money. They would still be spending time writing them, still using the same amount of ink and paper, and still delivering them. These costs stay the same.
Cleaely if you were to offer just a 32 page WT the donation would be smaller. Also when you are printing in the millions it makes the producing of one magazine a few cents (or pence in Britain) So big profit margin.