Best guess, a combination of "wait, we have a character named Wonder Woman?" and brand management - any version of Diana who showed up in Supergirl's story would obviously be markedly different than the one who's showing up in the Wonder Woman story. (Also, if the primary story arc is "these pets be crazy," having a character pop up who can both tell you exactly what's wrong *and* go on to calm them both down and fix it in the space of three panels sort of cuts your story short).