I think it's all of the above. But I have seen if advertised A LOT. The trailer was also attached to Avatar 2 and was extremely bad so a lot of people's first impression of the movie was that horrendous original trailer. Now Ant-Man 3's trailer was equally atrocious but I think that was buoyed the opening weekend thanks to Jonathan Majors/Kang.
Plus Levy is still in a muscle suit, the villains are who cares characters, the VFX look dime a dozen, And in my opinion this take on Captain Marvel where his immature kid personality is extremely at the forefront even when he's in his Captain Marvel form just doesn't work. Especially as Levy gets older his attempts to come across as young and youthful get more and more cringy. Dude looks normal when he is in his regular life with his regular colored hair and like a beard. But the jet black Captain Marvel hair and smooth face and probably heavy layers of makeup to make him seem younger are not resulting in an impressive visual.
And all that not so goodness is wrapped up in a big fat bow with the sticker on it that says "for the bin courtesy of James Gunn".
Let's be real if there was going to be another Shazam movie it would have to be because this one made like seven or eight hundred million dollars worldwide. And it would be at least 4 years before that materialized. Levy would be 46 doing his big kid routine. That works if you've got a level of talent like Robin Williams but not many people do. It's not a slight again Zachary Levy. It's just that ability is very rare and can't be taught.