Wonder Woman just seems to be a bit of an odd duck, really. Writers and fans loooove to contrast Batman and Superman, and Diana is just... Kinda' there. Maybe male writers (who dominate the genre) are intimidated by strong women on some level? Perhaps it seems like too much of a disconnect when she - as a mystical, mythic heroine - is set in the same narrative as a somewhat down-to-Earth pulp hero and science fiction icon? There's no great tragedy in her origin story, either; her island didn't exactly sink or explode, and Hippolyta wasn't shot by Ares or anything. We can easily define who she is, but not so easily what she stands for or how she acts on those beliefs in a way that's distinctly different from "the boys."
I mean... Superman's got the Secret CitadelFortress of Solitude and his Silver Age whimsy, Batman's got his intellect, issues, and motif... And Wonder Woman's got an invisible jet. Hmm.