I can see why they wanted to make a Shazam sequel at the time but if they weren't going to go the Black Adam route then it really made no sense. The Rock played a part in why both his own and this movie had lackluster buzz to begin with. But that doesn't take the creators off the hook, they could have made a better movie here,
All in all, WB was not going to bother marketing it more than they did. They knew there was no point.