Provide a possible explanation as to why the phenylenediamine reagent reduces cytochrome c oxidase and not other oxidases

It's due to the terminal oxidase system.

The cytochrome comprising species generate an intracellular oxidase enzyme. This oxidase enzyme catalyzes the oxidation of cytochrome c. The organisms; that comprise cytochrome c as part of their respiratory chain are oxidase positive and convert the reagent into blue or purple color.

The species lacking cytochrome c as part of their respiratory chain do not oxidize the reagent, leaving it colorless within the restrictions of the test, and are oxidase-negative.

The oxidase positive bacteria exhibits indophenol oxidase or cytochrome oxidase. Both of these catalyze the conduction of electrons from donor compounds (NADH) to the electron acceptors (generally oxygen). The test reagent phenylenediamine dihydrochloride functions as an artificial electron acceptor for the enzyme oxidase. The oxidized reagent produces the colored compound indophenol blue.