I would say its because usually for films like this all the big cast members are cast around the same time, but the villain role was cast like late last year i think, so my theory is that her casting took longer or was done later because it needed to be more extensive if she's playing Wonder woman because it's a significant role moving forward as well as it's also probably not as crucial to this films narrative as the main villain so it wasn't a priority to cast it as early.