All this nonsense with Gunn probably affected it. People know it doesn’t matter and so they are ignoring it.
Some would like to deny the disinterest in some of the last DCEU films now that they know it has been cancelled (as if there's some undying loyalty to a series ending in this way), but this emotional state is not at all uncommon: in the 1990s, many Burton Batman fans did not care about the then-continuing series when it was announced Burton and Keaton were no longer attached to the films. Some complained, while others lost all interest, since they believed "their real Batman" series was unceremoniously brought to its end.
Then, there's the Shazam factor: to numerous DCEU fans, this interpretation was never held in as high regard as the more Snyder-aligned films, so mass disinterest in the sequel is not breaking news. It says much when the only thing routinely praised in the film is Gadot's cameo. At least with another under-performing film-- Black Adam--there were months of teasing build-up for Cavill's long-desired return to the Superman role, but audiences did find other enjoyable elements, such as the JSA, for one example. Who is finding much to really enjoy in the Shazam sequel?