And even when they're revealed it might happen that it's actually not what they announced first. Remember Julias Caesar's additional Military Policy card slot when conquering its first capital, or Mbande Nzinga's ability toward trade routes much more engaging that the watered down Lady Six Skies?
It wouldn't surprise me if those problems are the reason we don't see the abilities early. It could be that the social media team needs to build their previews earlier than they get confirmation that the abilities won't change. So it's just safer to not build the previews with abilities.