Several reasons, but first some background. A ringdown circuit is a box that emulates a phone company for two phones on a point-to-point basis. For example, imagine a phone in a shop directly connected to a supervisor's office. A ringdown circuit allows either end to automatically ring the other end when the handset is picked up with no dialing required.
As you can see, this mode of operation isn't the point of the field phone system at all. The field phone system supports many phones on two lines (or one line if conferenced or bridged together), and with a deliberately silent call indication or quiet piezo. In our zombie protection application, we don't want the normal phone ring. See the ring interface module question for more details about that.
Also, a ringdown circuit controller is typically more expensive than the field phone, certainly once this campaign succeeds! Ringdown circuits also typically burn more power and lack the wide charging options of the field phone system.