Because I think that's a pretty powerful message to lose for mere astrology.
I don't think it's mere astrology. Diana was made by the Gods, who traditionally dwell in the sky, so I do think it's very fitting for her to "wear the sky".
Honoring a fallen hero is a powerful message, but it's very common in the DCU; plus, it adds awkward baggage (ok, so when Diana Trevor died to help the Amazons, you started wearing her flag to honor her. Are you going to start wearing the symbols of all the heroes who died to save the world and thus indirectly died to same Themiscyra?). Whereas being literally god-made is unique and baggage-free, so I'd much rather her costume focus on that.