Because the standard passenger constant-flow mask primarily has the bag so that the FAA requirement of flow indication can be satisfied, it is not needed because the EDS provides a positive indication of oxygen being supplied as well as an additional alert to indicate otherwise

The EDS system is an SBA (Supplementary Breathing Apparatus) class Active Phase Dilution system. That is it actively supplements the proper amount of oxygen and air at the proper phase dilution at the various points in your breathing cycles without the need of extra check valves in the mask.